Painting over Jotamastic 90

[Roy Steiner]

I have painted my bilge with Jotamastic 90 following excellent advice from my previous posting.  The advice was spot on and I am well pleased with the result.  I have some areas such as a foot well leading into the cabin and the gas locker with corrosion problems and after thorough preparation I would like to coat it with the same product followed by HMG coach enamel.  It does advise in it's datasheet that it is suitable as a primer.  In your experience, can I paint straight onto the Jotamastic, have a different paint such as a grey primer in between, apply while still not fully cured or wait for full curing and sand lightly before painting and indeed any information I have not yet considered.  I have looked at the datasheets etc but they just want to plug their own products which are not in the right colour for my needs and I suspect dealers will feel obliged to give the same information as in the datasheet. 

[David Mack]

Whatever they say about overcoating with their own brand single pack paints will be equally applicable to other brands of the same type of paint.

[Roy Steiner]

Thanks.  Their alternatives are either an acrylic paint, a two pack polyurethane or a two pack epoxy but that is not surprising as I believe they are their only topcoats.  I have not found any enamel topcoats other than a domestic enamel paint that would not be suitable as a coach enamel.  I therefore cannot be sure if their advice is bound by what they make or the paint is actually unsuitable for enamel topcoats.  I am sure I have read that you can paint over it but that is less than anecdotal.  I therefore will need to rely on user experience, hence my post.

[ditchcrawler]

 

[Bee]

The hull of Bee is painted in Jotumastic to a few inches above the waterline, to make a nice neat job of it the hull above the waterline is painted in bitumastic. The bitumastic has flaked off the jotul and it now looks awful so I guess that painting over Jotul means making sure that it is given a good roughening so ordinary paint has a decent key. 

[dmr]

Wait till the Jotun has fully set, its reactions are totally different to single pack so painting before the Jotun is fully cured has no advantage, and possibly big disadvantages. Sand the Jotun to slightly roughen it and paint over the top. This is how our tunnel bands are done and they have lasted for years.

[Roy Steiner]

32 minutes ago, ditchcrawler said:

 

Thanks, may have an answer there.  blackrose  suggested Hempel primer undercoat and looking at the datasheet it is suitable to paint on fibreglass so would perhaps be suitable as an undercoat and the Hempel advice booklet suggests can be overpainted with one part gloss.  I will await further pearls of knowledge so please keep them coming.

[blackrose]

46 minutes ago, Roy Steiner said:

Thanks, may have an answer there.  blackrose  suggested Hempel primer undercoat and looking at the datasheet it is suitable to paint on fibreglass so would perhaps be suitable as an undercoat and the Hempel advice booklet suggests can be overpainted with one part gloss.  I will await further pearls of knowledge so please keep them coming.

 

I think that undercoat will be suitable for fibreglass but check the TDS. Just to clarify, the advice I gave in the linked thread was regarding painting above the waterline. Hemple/Blake's primer/undercoat isn't suitable for applications under the waterline or bilge areas prone to being constantly immersed.

Edited October 26, 2023 by blackrose

[Roy Steiner]

Sorry, I may have misled you there.  The reference to fibreglass was merely that it overlapped paint systems, the paint surface I will be working on is Jotamastic 90 which is what you were referring to in the previous topic.  Thanks for being so perceptive and taking the time to check on my post 👍

 

My bilge I have left as Jotamastic topcoat as it does not have direct sunlight

 

Edited October 26, 2023 by Roy Steiner

[dmr]

You can get Jotamastic in any colour that you want, so a light grey would be nice in the bilge.

 

I stuck the tunnel bands (in Toplac) straight over the Jotun, but if you feel the need for a "tie coat" then I find Bondaprimer works well in most cases, though have not yet tried in over Jotun but have used it over other epoxies.

Edited October 26, 2023 by dmr

[Roy Steiner]

14 minutes ago, dmr said:

You can get Jotamastic in any colour that you want, so a light grey would be nice in the bilge.

 

I stuck the tunnel bands (in Toplac) straight over the Jotun, but if you feel the need for a "tie coat" then I find Bondaprimer works well in most cases, though have not yet tried in over Jotun but have used it over other epoxies.

My bilge is already painted with the Jotamastic and in light grey as you suggest.  Great minds think alike although my mind is probably not so great.  My issue is painting the footwell and gas locker which I would like in the boat colour, and will be exposed to UV, hence a topcoat.

 

I presume you are referring to Bonda Rust primer; I see from the data sheet that it is an alkyd resin; not sure of compatibility with Jotamastic 😞

 

 

Edited October 26, 2023 by Roy Steiner

[dmr]

The gas locker won't see much light, the Jotun will be fine. What do you mean by footwell? if its somewhere that you walk (welldeck?) then gloss will be much too slippery

[Roy Steiner]

It is a step down into the cabin.  Unfortunately it is below water level so water can collect there if drainage blocked.  I am fixing up better drainage with a pipe so it goes directly to where the bilge pump is.  It usually has a small wooden deck in it so the gloss will be fine.  Owing to historical blockages and retained water, the corrosion needs addressing, fortunately not a welding job but a few more years of similar suffering may lead to such a need, hence a belt and braces job!

[blackrose]

11 hours ago, Roy Steiner said:

My bilge is already painted with the Jotamastic and in light grey as you suggest.  Great minds think alike although my mind is probably not so great.  My issue is painting the footwell and gas locker which I would like in the boat colour, and will be exposed to UV, hence a topcoat.

 

I presume you are referring to Bonda Rust primer; I see from the data sheet that it is an alkyd resin; not sure of compatibility with Jotamastic 😞

 

The only issue with Jotamastic 90 and sunlight is that it will chalk and turn a lighter colour. It won't be an issue inside a gas locker but if you're talking about the outside of the locker then either use Hemple/blakes primer-undercoat over your Jotamastic 90 and then single part topcoat of your choice, or consider using Jotun Hardtop directly on top of the Jotamastic 90. It's a two part epoxy topcoat available in a variety of colours which isn't affected by UV. 

 

Speak to SML or another supplier first as I've never actually used Hardtop myself. As usual, pay attention to the minimum/maximum overcoating times in the TDS for best adhesion between coats.

 

I've had mixed results with Bondaprimer. I used it in the past with good results but then I kept getting cans that became like gel in the can - even brand new cans, and they wouldn't thin with cellulose thinners, so in the end I stopped using it. I should think it's compatible on top of Jotamastic 90, however I also know though bitter experience that (like any primer) Bondaprimer will need an undercoat between it and any topcoats. So then you're back to square one.

 

11 hours ago, dmr said:

 

I stuck the tunnel bands (in Toplac) straight over the Jotun, but if you feel the need for a "tie coat" then I find Bondaprimer works well in most cases, though have not yet tried in over Jotun but have used it over other epoxies.

 

I really don't think that Bondaprimer is a good tie layer, a decent single part undercoat between the Jotamastic and the topcoats will be much better in my experience. Years ago I was told by the manufacturer of Bondaprimer that I could paint topcoats on directly without an undercoat. That turned out to be very bad advice as my topcoats slowly peeled off. I followed the (very limited) Bondaprimer instructions but I realised afterwards that I should have used an undercoat between which I have done ever since and never had that problem again. 

 

Now I always follow the mantra: primers/undercoats/topcoats

 

That particular balls up on the cruiser stern bulkhead meant I either had to take those peeling topcoats off or take the whole lot back to bare steel. I chose the lazy option leaving the Bondaprimer layers in place as they were well adhered to the steel, but of course after I'd finished getting the topcoats off with the sander the finish was so uneven that when I painted it in gloss grey it looked terrible. I ended up buying some mat grey topcoat to mask the issue and it's amazing the multitude of sins that a mat finish will hide, but the problem with mat paint is that it marks very easily and needs overpainting every year! These mistakes can have repercussions for a long time... One day I'll take it all back to metal and start again.

Edited October 27, 2023 by blackrose

[Roy Steiner]

Thanks blackrose.  I will definitely be going for Hempel primer/undercoat unless anyone else has different experience.  I suspect this posting has now run dry but since the weather is not good for painting I will wait a short while before proceeding. 👍