Undercoating an epoxy primer

[Mark R]

Morning all,

 

I've finally primed my boat with 4 coats of Jotasmastic 87. This has been rolled on creating a coating thickness of around 200 microns. It has left an orange peel texture. 

 

My question is, should I sand this down to form a smoother surface for the undercoat, reducing coating thickness of the primer, or should I overcoat it with a number of coats of undercoat and then sand this down to a smooth surface, this would obviously also affect the combinded surface thickness but would be easier to do. I coud always apply another coat or two of undercoat on top of this.

 

Any thoughts would be appreciated,

 

Mark

[matty40s]

I would protect your primer with high build undercoat and sand flat afterwards. You can always add another undercoat afterwards using brush to lay off if any primer shows through 

[Mark R]

Thanks @matty40s, that was my way of thinking too. One question if I may, prior to applying undercoat, I'd give a light sand of the epoxy to rough it up and get rid of any high peaks or imperfections, should I be worried about the undercoat not sticking to the pits of the orange peel epoxy as they haven't been touched by the sanding. 

[matty40s]

Yes, always good to get a key on it.

[dmr]

4 coats is plenty, Jotun say 2 is ok above the waterline, so you should have plenty of thickness.

 

A lot depends on how rough your surface is. It can take a LOT of coats of undercoat to cover significant roughness.

A sand of the Jotun to take the shine off is a good idea. When you sand and dust off you will see how rough it is, the peaks will have sanded but all the pits will still be a bit shiney. I reckon carefully sand till you have taken att the peaks/high spots almost totally away, then undercoat.

 

and if put a coat of undercoat on first its even easier to see what the sanding is acheiving.

[Mark R]

Thanks @dmr, sounds like I have enough coating to play with. I just need to be careful when sanding those curved edges!

[blackrose]

On 11/06/2025 at 07:33, Mark R said:

Morning all,

 

I've finally primed my boat with 4 coats of Jotasmastic 87. This has been rolled on creating a coating thickness of around 200 microns. It has left an orange peel texture. 

 

My question is, should I sand this down to form a smoother surface for the undercoat, reducing coating thickness of the primer, or should I overcoat it with a number of coats of undercoat and then sand this down to a smooth surface, this would obviously also affect the combinded surface thickness but would be easier to do. I coud always apply another coat or two of undercoat on top of this.

 

Any thoughts would be appreciated,

 

Mark

 

Four questions:

 

1) When you say you've primed your boat with Jotamasic 87 can we assume this has been applied to the topsides? If it was below waterline hull then you don't need undercoat.

 

2) Is your undercoat single part or two part?

 

3) Have you read the full Technical Data Sheet for Jotamastic 87? I think if you get the undercoat on within a certain maximum overcoating time you don't need to key the Jotamastic 87 - unless you're just doing that for a smoother surface and to get rid of the orange peel.

 

4) Why are you using Jotamastic 87 rather than Jotamastic 90?

 

 

[Mark R]

@blackrose, thanks for you comment. Yes the primer is above water level, the undercoat will be single part. I've read the technical data sheet several and have spoke to Jotun 3 times regarding the product too, one of which was to enquire about the differences between 87 and 90. Jotamastic 87 was chosen as it was in stock at the time and improvements of 90 weren't so critical given it's intended use. In terms of painting over the epoxy I'm aware that I have a bit of time before needing to key the surface, that's no concern, the peaks are more the reason for giving it a sand.

[dmr]

41 minutes ago, Mark R said:

Thanks @dmr, sounds like I have enough coating to play with. I just need to be careful when sanding those curved edges!

Yes I forgot to say thay, even epoxy goes through very easily on an edge, if it all goes wrong you can always put a bit more Jotun on😀. If you use Jotun 90 with wintergrade hardener you can easily do two coats on the same day at this time of the year.

 

Just to get my poster credibility, I have done similar myself quite a few times but only on very local repairs, I don't think I would fancy sanding a whole boat. Will be doing a bit of epoxy sanding and an undercoat later this morning. Note the use of light grey Jotun 90.

40 minutes ago, Mark R said:

@blackrose, thanks for you comment. Yes the primer is above water level, the undercoat will be single part. I've read the technical data sheet several and have spoke to Jotun 3 times regarding the product too, one of which was to enquire about the differences between 87 and 90. Jotamastic 87 was chosen as it was in stock at the time and improvements of 90 weren't so critical given it's intended use. In terms of painting over the epoxy I'm aware that I have a bit of time before needing to key the surface, that's no concern, the peaks are more the reason for giving it a sand.

Im not sure about the time delay before keying the surface as its not like for like.

 

Both epoxy and Alkyd paints take a while to fully set so a second coat will chemically bond to the previous one during this time. After a time keying is needed to give a rougher surface to help physical bonding as there is no chemical bonding. The single pack/alkyd undercoat will never chemically bond to the epoxy so a bit of sanding is always a good idea.

 

I agree with your comment that Jotun 90 is not that different to 87. The availability of 90 in any colour you fancy is possibly its biggest advantage.

 

Edited Thursday at 07:32 by dmr
typo

[Mark R]

Thanks guys, are any of you able to suggest a suitable paint that can be used as an undercoat over the Jotamastic? The reason I ask is because I happened to go to Manor Coatings, a local paint manufacturer (recommended by a boat painting business) yesterday to discuss which of their products might be used. I was told that I'd need a 2 pack undercoat to go over epoxy and that they couldn't recommend any of their products. This led me to phone Jotun and they recommended their Pilot II paint (an alkyd based, single system paint), I felt like the guy I spoke to wasn't really that experienced as he had to discuss this and another question with a colleague, he did invite me to send an email and I'd get a more detailed response - I'll post that here when it comes. Pilot II appears to be a top coat and not an undercoat, so not sure if this is wise. Two undercoat paints I've seen are:

 

https://www.symphonynarrowboatpaint.co.uk/undercoats/undercoat-grey.765382

https://paints4trade.com/rapidguard--universal--primer-undercoat-paint-1094-p.asp?_=&variantid=274786

[matty40s]

We use normal high build Symphony single pack undercoat over 2 pack. We've been priming in 2 pack for years now and never had an issue.

[blackrose]

1 hour ago, Mark R said:

Thanks guys, are any of you able to suggest a suitable paint that can be used as an undercoat over the Jotamastic? 

 

Hempel Primer undercoat is a good one.

 

On 12/06/2025 at 08:30, dmr said:

Im not sure about the time delay before keying the surface as its not like for like.

 

Both epoxy and Alkyd paints take a while to fully set so a second coat will chemically bond to the previous one during this time. After a time keying is needed to give a rougher surface to help physical bonding as there is no chemical bonding. The single pack/alkyd undercoat will never chemically bond to the epoxy so a bit of sanding is always a good idea.

 

I'm not sure you're correct here. Presumably by "time delay" you mean the maximum overcoating time? Jotun do publish figures in their TDS for maximum overcoating times when painting other types of paint on top of Jotamastic including acrylic and polyurethane paint.

 

The maximum overcoating times given by Jotun relate to the "Maximum time before thorough surface preparation is required" (as quoted from the previous page to the image below). So while I'm not sure about alkyd paints, single part paints will chemically bond to epoxy if applied within these times 

 

 

In any case the OP is going to key the surface for smoothness rather than bond strength so it's a moot point.

Edited Friday at 08:28 by blackrose

[dmr]

1 hour ago, Mark R said:

Thanks guys, are any of you able to suggest a suitable paint that can be used as an undercoat over the Jotamastic? The reason I ask is because I happened to go to Manor Coatings, a local paint manufacturer (recommended by a boat painting business) yesterday to discuss which of their products might be used. I was told that I'd need a 2 pack undercoat to go over epoxy and that they couldn't recommend any of their products. This led me to phone Jotun and they recommended their Pilot II paint (an alkyd based, single system paint), I felt like the guy I spoke to wasn't really that experienced as he had to discuss this and another question with a colleague, he did invite me to send an email and I'd get a more detailed response - I'll post that here when it comes. Pilot II appears to be a top coat and not an undercoat, so not sure if this is wise. Two undercoat paints I've seen are:

 

https://www.symphonynarrowboatpaint.co.uk/undercoats/undercoat-grey.765382

https://paints4trade.com/rapidguard--universal--primer-undercoat-paint-1094-p.asp?_=&variantid=274786

 

So Jotun are saying that a single/alkyd paint is fine over epoxy and, as you would reasonably expect, are suggesting their own brand rather than Craftmaster/Toplac/Several others. As I have said elsewhere, Joton do both Pilot (big pots) and Lustre(small pots/more expensive) and SML said that Lustre is better for brush/roler and Pilot is better for professional use/spraying. This might be true or might just be a marketing thing.

 

I like Craftmaster high build undercoat but I suspect any reputable undercoat would be fine, and Matty does painting much more seriously than I do. I would feel safer using boaty type paints rather than decorators paints unless other with first hand experience give a recomendation. 

25 minutes ago, blackrose said:

 

Hempel Primer undercoat is a good one.

 

 

I'm not sure you're correct here. Presumably by "time delay" you mean the maximum overcoating time? Jotun do publish figures in their TDS for maximum overcoating times when painting other types of paint on top of Jotamastic including acrylic and polyurethane paint.

 

The maximum overcoating times given by Jotun relate to the "Maximum time before thorough surface preparation is required" (as quoted from the previous page to the image below). So while I'm not sure about alkyd paints, single part paints will chemically bond to epoxy if applied within these times 

 

 

In any case the OP is going to key the surface for smoothness rather than bond strength so it's a moot point.

 

Yes, I saw that as well and thought it a bit strange, but we have to hope that Jotun know their stuff. I took the opposite view and was a bit concerned about putting single pack over two pack before the two pack was fully cured, but looks like its fine, it hasn't fallen off yet  😀

The aphids or whatever are having a bumper year so an additional problem for me is cleaning the sticky mess off the boat between coats, and the sticky stuff extends a fair way beyond the over hanging trees.

[Mark R]

Thanks guys, as ever you've all been so helpful. I'm hoping to have the topcoat done by boat painters as they have a two week period available in their paint tunnel and although I can use the tunnel myself I think the finish they'd achieve would be far, far better than I could expect by doing it myself. As time is so limited I'm wanting to do as much as I can myself and deliver it to them in a state that they might just manage to complete the task in the time available. 

 

They typically use 'Manor Coatings' paints but are happy to use any paint I supply or request. Living only a couple of miles from Manor Coatings I went down for advice but they've told me that their products are a no go over epoxy. Craftmaster and Symphony were the two makes that stood, typically through discussions on here. Looking at the datasheet for Symphony, I assume that polyurethane paint is a type of alkyd paint

 

@blackrose, interestingly the variety of paint types mentioned in application guide for Jotun 87 (non Aluminium) are less and the interval durations differ to that of Jotamastic 87 (Aluminium) and 90 (Aluminium). @matty40s, given the 'maximum overcoating intervals' I show below, I won't get the undercoat on within that period. Let's say I sand off the peaks, there are still going to be areas that haven't been abraded on the primer coat, is this an issue, if so should I maybe look at sanDing to a flatter finish or focussing on getting a coat or two of undercoat on ASAP?

 

 

[blackrose]

11 minutes ago, Mark R said:

 Let's say I sand off the peaks, there are still going to be areas that haven't been abraded on the primer coat, is this an issue, if so should I maybe look at sanDing to a flatter finish or focussing on getting a coat or two of undercoat on ASAP?

 

 

 

If you're sanding the orange peel down then I wouldn't worry too much about the "valleys" between the peaks. I'd probably just go over the sanded surface with some green Scotchbrite. 

 

I've used single part Hemple Primer Undercoat on top of Jotamastic before and it goes on well. But I'm wondering if you're going to all the trouble of epoxying the topsides of your boat and then going to the expense of having it professionally painted, why wouldn't you use two-part top coats for extra durability? 

 

I don't know much about two part-top coat paints, but a boat painter I know uses Epifanes two part undercoat and top coats. It might be worth looking into if your painter has experience using them. 

 

The other thing I'd say is that you're obviously doing your due diligence by thinking things through and reading the technical data sheets for the paints. Just make sure the painter is doing the same and understands minimum/maximum overcoating times, etc. You'd be surprised how many don't. If you suspect they're not technically-minded then perhaps stick to single-part undercoat/topcoats.

[Mark R]

Thanks @blackrose, in terms of sanding the orange peel down, I wasn't going to attempt to achieve too flat a finish, as @matty40s suggested, I'd use the undercoat to fill the valleys, I'm assuming that the prep you discuss is suggesting that too (i.e. not going too smooth with the epoxy sanding), sanding the peaks and finishing off with a rub over of Scotchbrite?

 

It's an interesting thought regarding a 2 pack topcoat, for some reason I just thought the 'done thing' was to use traditional topcoat paints on narrowboats. I can ask the question when the quote comes through regarding the work. I'd love a deep shine to the finish, is this achievable with two pack systems?

 

On a side note, I had thought of painting the roof of the boat myself as the finish on this isn't so important, given that it'll be covered with solar panels, accessories etc, and that it's pretty somewhat pitted anyway. In which case maybe I could consider a two pack paint that might provide better durability!

[dmr]

The (only?) advantage of a single paxk topcoat is that it will be easier to make small repairs in the future.

[Mark R]

24 minutes ago, dmr said:

The (only?) advantage of a single paxk topcoat is that it will be easier to make small repairs in the future.

 

@dmr, easier in as much as just not having to mix a two pack paint or easier to 'blend' in?

 

Excuse my ignorance but what do single and two pack paints fall under, I'm assuming some are oil based, some epoxy, but others?

Edited yesterday at 08:53 by Mark R

[dmr]

Ive not used two pack topcoats so really should shut up 😀, single pack must be easier to work with, though mixing very small quantities of jotun 90 is relatively quick and easy with a cheap set of drug dealer scales so topcoat should be similar. I suspect brushing anything but a small area will need more skill with two pack topcoat.

 

One pack paints are a complicated subject, they are referred to as oil based but almost all are alkyd (some chemical) but then modified with silicone or polyureEhane to give slightly different advantages. Some are sold as polyurethane, not sure if these are a modified alkyd or just polyurethane.  Most of the paints are much of a muchness. I suspect Craftmaster, symphony, Toplac etc are all very similar, but a few are significantly different. I think epifanes is a different chemistry, and Rylands (now hard to get) might be a bit different.

As I keep saying though, I am only a boater/very amateur third rate painter. 

[agg221]

34 minutes ago, Mark R said:

 

@dmr, easier in as much as just not having to mix a two pack paint or easier to 'blend' in?

 

Excuse my ignorance but what do single and two pack paints fall under, I'm assuming some are oil based, but others?

A single part paint cures by evaporation of solvents and often also by a chemical reaction with air or water. The chemical bonds formed within the paint are not very strong, so it is relatively easy to wear and scratch it.

 

A two part paint cures by chemical reaction between two different chemicals which are mixed together just before painting. Once the reaction happens, it is not reversible. This means the chemical bonding in the paint is much much stronger so the paint is hard and resists scratches. The fact that the paint is a resin means it only needs a small amount of solvent to thin it, rather than quite a lot of solvent to carry it (some oil based single pack paints also do this). This also typically means it cures to a much denser film so air and water don't get through so easily, and some can also chemically react with the metal surface, blocking the surface from further chemical reactions with air and water, ie they stop it from rusting.

 

Epoxy and polyurethane are the two types of two part (two pack) paint you are most likely to encounter. They both work as above, but they are chemically very different and the chemical reaction that happens is not the same. Epoxies mostly cure with amines (the slightly fishy smelling part b) and polyurethanes mostly cure with isocyanates which smell like superglue.

 

Performance-wise, a two part paint is much better. It is harder and tougher and protects the surface better so the paint does not flake off if you get a scratch through. However, mixing tiny quantities for spot repairs is difficult (not impossible). More importantly, whilst epoxies seem to last forever in the tin, polyurethanes (topcoats) are moisture sensitive and 'go off' in the tin. That makes it very difficult to deal with scratches over the years you need the paint to last. There are some choices on compromise - you have already covered the most important protection with an epoxy primer so you could now use a single part undercoat and topcoat. This would make it easiest to touch up but it will need doing more often, or you could use an epoxy undercoat and single part topcoat (alkyd or mono-urethane) which will mean the scratches tend to get less deep and the colour part is still easy to touch up, or you could use an epoxy undercoat and a polyurethane topcoat (I would also second the Epifanes recommendation - boatpaint.co.uk had the best price last time I bought some and I have used them several times, finding them very helpful in mixing to RAL colours), using a colour scheme where the exposed areas are a different colour, so for example framing out the panels and painting the hand rails. These small areas would then be painted in a single part paint so they are easy to touch in, while the main sides which are less exposed are in the two part paint for durability.

 

Alec

15 minutes ago, dmr said:

One pack paints are a complicated subject, they are referred to as oil based but almost all are alkyd (some chemical) but then modified with silicone or polyureEhane to give slightly different advantages. Some are sold as polyurethane, not sure if these are a modified alkyd or just polyurethane.  Most of the paints are much of a muchness. I suspect Craftmaster, symphony, Toplac etc are all very similar, but a few are significantly different. I think epifanes is a different chemistry, and Rylands (now hard to get) might be a bit different.

As I keep saying though, I am only a boater/very amateur third rate painter. 

The most uncommon chemistry, which I am currently trying out to see how well it performs in practice, is the polysiloxane cured epoxy. In theory it doesn't matt, chalk and fade like epoxy normally does. For now, single part alkyds and mono-urethanes, and two part polyurethanes appear to be the best options for topcoats.

 

One other thing to mention - pigment is expensive, resin is cheap. It is fairly obvious therefore how different paint brands can be sold at different prices. Paint really is one of those things where you get what you pay for, typically more pigment in the better brands. This gives better colour coverage, lower permeability to air and water and a stronger paint, particularly true in a single pack paint where the resin is not as strong. There is therefore a very good reason to use a brand based on recommendation. FWIW, International, Hempel, Jotun and Epifanes all spring to mind immediately as high quality brands. There are others too such as PPG Sigma but they are more niche, but what it does mean is if you spot something locally that appears to be a good deal, from an unknown brand, it is worth checking online whether that is an uncommon high quality brand or something cheap and cheerful.

 

Alec

Edited yesterday at 09:18 by agg221

[dmr]

12 minutes ago, agg221 said:

 

One other thing to mention - pigment is expensive, resin is cheap. It is fairly obvious therefore how different paint brands can be sold at different prices. Paint really is one of those things where you get what you pay for, typically more pigment in the better brands. This gives better colour coverage, lower permeability to air and water and a stronger paint, particularly true in a single pack paint where the resin is not as strong. There is therefore a very good reason to use a brand based on recommendation. FWIW, International, Hempel, Jotun and Epifanes all spring to mind immediately as high quality brands. There are others too such as PPG Sigma but they are more niche, but what it does mean is if you spot something locally that appears to be a good deal, from an unknown brand, it is worth checking online whether that is an uncommon high quality brand or something cheap and cheerful.

 

Alec

 

I am using Jotun 90 in a light grey colour. I note that the price from SML is a base price plus an additional cost based on the chosen colour. 

 

as for mixing two pack, I am confident about mixing small amounts of two pack (Jotun 90). Half a shot glass is about my minumum, and collecting the shot glasses can be fun 😀 In addition to the digital drug dealer scales, part B might need a bit of warming to pour nicely, and needs decanting into a small container that is easy to pour from. An old glass spice jar is good, (and not filled to the top). As quantities get smaller it obviously gets a bit harder to do an accurate mix.  Mixing also needs care to make sure the stuff stuck to the sides of the shot glass gets properly mixed in etc.

and don't mix outdoors on a windy day.