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Boat Painting - Sand Between Each Coat?


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We're just about to start painting our boat! It's primed by the builders in 2-pack, and we've bought all our paint from Bailey's!

 

Would you sand between each coat of undercoat and top-coat, or lay a couple of coats before flatting? I'm thinking of 400grit for the undercoat...any thoughts?

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Hi there,

The rule of thumb for me is to apply each coat with as much care and attention as the finished gloss, by that I mean with little or no brush/roller marks. This is best achieved on a mild day with very little if any breeze or better still in a wet/dry dock.

The better the paint is applied the less flatting back will be needed so the grit rating partly depends on how well it was put on in the first place..

Generally.

240grit for primers and undercoats

500grit for glosses

800 to 1000grit before the final top coat.

If undercoat or primer has been applied and a second coat is required, providing no more than 24 hours have lapsed then you can avoid flatting back this coat. The original coat will slightly soften and bond to the second coat.

I wish this also applied to glosses but unfortunately it does not. In fact over many years I have realised that a newly flatted gloss is better left for 24 hours before another coat is applied.... Flatting back removes the top harder surface on new paint thus leaving a softer paint underneath that can occasionally react with the new coat, especially if it is a horizontal surface and the paint is being applied thickly.

Clear as mud?

Sorry.

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It astonishes me the fine grits people seem to think are required for rubbing down applied paint. If you want a finish like that on a Rolls Royce or one of her majestys horse drawn coaches then using 500 grits and above would be warranted. In the motor trade, respraying cars final prep is generally around 400 Commercial vehicles 280 grit to 300 grit.

 

For brush spray or roller undercoat I use 120 to 150 grit on early coats and prior to gloss 280 grit. nowhere on the boat have I used more than 400 grit and that only in sprayed areas, and I can guarantee you won't find rubbing down marks in my paintwork, even in the sprayed areas. All using fine grit paper on brush finished general painting creates is un-necessary hard work and in many cases inappropriate keying for good adhesion of some paints.

 

We used to use 1000 grit on finished paint prior to polishing, it;s far to fine for general preparation even final coat.

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We're just about to start painting our boat! It's primed by the builders in 2-pack, and we've bought all our paint from Bailey's!

 

Would you sand between each coat of undercoat and top-coat, or lay a couple of coats before flatting? I'm thinking of 400grit for the undercoat...any thoughts?

 

Best not to add any sand at all between coats, unless you want a non-slip surface.

 

Hope that helps.

 

 

mtB

  • Greenie 1
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i am doing the lockers and have two coats of primer on.should i sand before the top coats or just lash it on?

 

If the primer is newly applied all that's needed is a basic de-nib with something around 100 grit to say 150 grit. This is not necessarily to rub the primer down, de-nibbing removes any bits of dust speckles before applying the next coat. If you've applied any fillers, of course these need rubbing down.

 

Ideally You'll need to apply undercoat prior to top coat.

 

However if you primer has been on the steel for a long period, more than a couple of days leading into weeks, then it will need to be fully rubbed down, especially if exposed to the sun we've had recently. So re keying the surface would be required. Most primers need to be recoated quickly, especially 2 pack primers, if these are left beyond manufacturers re-coat time then a full rubwown is needed again 150 grit will give an excellent key.

 

Also if you're doing any painting and prep then invest in an Orbital sander, it will save hours in rubbing down. Even cheap ones will do but might not last. I bought a Wicks one second hand on e-bay for a tenner, done the whole boat and much more and it's still going strong.

Edited by Julynian
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If the primer is newly applied all that's needed is a basic de-nib with something around 100 grit to say 150 grit. This is not necessarily to rub the primer down, de-nibbing removes any bits of dust speckles before applying the next coat. If you've applied any fillers, of course these need rubbing down.

 

However if you primer has been on the steel for a long period, more than a couple of days leading into weeks, then it will need to be fully rubbed down, especially if exposed to the sun we've had recently. So re keying the surface would be required. Most primers need to be recoated quickly, especially 2 pack primers, if these are left beyond manufacturers re-coat time then a full rubwown is needed again 150 grit will give an excellent key.

 

Also if you're doing any painting and prep then invest in an Orbital sander, it will save hours in rubbing down. Even cheap ones will do but might not last. I bought a Wicks one second hand on e-bay for a tenner, done the whole boat and much more and it's still going strong.

thanks for that.

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I'm sanding a new-build, which has a lot of fake rivets on it!

I've been nibbing/keying the 2-pack primer with 220 (which is probably too fine) and working the edges and rivets, corners, etc with green-grade scotch brite, which is an excellent product for keying where sandpaper would catch 'edges' and take it back to bare metal!

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It astonishes me the fine grits people seem to think are required for rubbing down applied paint. If you want a finish like that on a Rolls Royce or one of her majestys horse drawn coaches then using 500 grits and above would be warranted. In the motor trade, respraying cars final prep is generally around 400 Commercial vehicles 280 grit to 300 grit.

 

 

Thing is that I'm assuming Tony Martin works hard to create a finish befitting of someones pride and joy, not an argos lorry. :)

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Thing is that I'm assuming Tony Martin works hard to create a finish befitting of someones pride and joy, not an argos lorry. smile.png

 

Well that was my point. Telling Jo blogs who just wants to get a reasonably decent finish on their boat to use such fine rubbing down papers is misleading. These type grits are used for coachwork and fine polishing not painting boats.

 

The finish on an Argos lorry or average finishe on commercial vehicles is probably far superior to the finish on 90% of boats currently on the water. Good preparation is important, but using a rubbing down grade that you're never going to match with a brush finish in nonsensical.

 

For people undertaking their own painting who are not skilled painters, using any abrasive over 300 grit is a complete waste of time and effort. Time would be much better spent applying lots of paint and polishing back.

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Thanks "Kitman"

 

I am sure "Northwich trader" would prefer Rolls Royce to Argos lorry too !

I see nothing wrong in attempting to achieve an immaculate finish hence my advice, If it falls short of Rolls Royce standard then isn't BMW standard still a good place to be ?

 

 

Tony.

www.boatpainters.co.uk

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Thanks "Kitman"

 

I am sure "Northwich trader" would prefer Rolls Royce to Argos lorry too !

I see nothing wrong in attempting to achieve an immaculate finish hence my advice, If it falls short of Rolls Royce standard then isn't BMW standard still a good place to be ?

 

 

Tony.

www.boatpainters.co.uk

Absolutely! I'm in the fortunate/unfortunate place where the boatyard says 'it's a replica working boat, so doesn't need the RR finish,' yet the world around me will say 'he couldn't pull off a classy finish, so he's calling it a working finish'!

Deep down, working boat or not, I want a finish that people AND myself can look at and say 'not bad...not bad at all'! :)

 

Edit: oh, should perhaps add...just completed 18 hrs of prep for the main and boatman cabins, undercoat tomorrow! :)

Edited by NorthwichTrader
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Just out of interest (I guess I could just call them tomorrow) but, does anyone know the minimum recoat time between undercoats?

I'm using Bailey's paints, and it would suit me perfectly to get two coats on in the same day, if it were possible?

Does it not say on the can. If its an ordinary oil based undercoat I should imagine it would be the normal 12 or so hours before recoating.

Edited by bizzard
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Just out of interest (I guess I could just call them tomorrow) but, does anyone know the minimum recoat time between undercoats?

I'm using Bailey's paints, and it would suit me perfectly to get two coats on in the same day, if it were possible?

Don't know about 2 coats in the same day.

Give there Technical help line a call and seek there advice.

The bloke from International paints was very helpful last week and said the best thing is to paint on consecutive days - that will allow time for a few cups of tea along the way.

The other thing he did say is to make sure you buy all the topcoat from the same batch number as there can be variations within the same colour.

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The other thing of course is applying the stuff, as mentioned, every coat should be applied with as much care to avoid brush marks as poss. It`s not easy to get a finish without brush marks, I lay it off in two directions to get rid of marks but it simply is not possible if the temperature is too high or if the brush is full of bits, use a brush of decent size. .I sometimes use cheap brushes and throw them away, others will disagree. Painting, like welding, is a skill, I can do both to my satisfaction and pocket but if you want the best it will cost.

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Thanks all! I didn't realise until today that the top of a 70ft boat with handrails on posts a foot apart, and the rivets replicating a folded over roof, were going to be such a challenge!

I could barely paint a foot-wide band across the roof, not in today's temperatures, without the paint dragging before I passed with my second band (and this was thinned with spirit too!)!

I learned a lot today! I'll have a bit of flatting back to do, but I'll be better next time around, now I know where all the 'catches' are!

 

Just out of interest, what's the accepted strategy for undercoats...multiple coats, or just a flattened coat that does the trick, and go for multiple topcoats instead?

Edited by NorthwichTrader
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First off, painting in this weather is madness, you need to be in full shade at least, and even then I would think twice.

 

You only need to rub down undercoat if it's needed and not been left too long. Assuming a fairly smooth coat has been applied, the most rubbing down needed is a denib, thats a cursory flick over with something like 300 grit to remove small marks/flecks sometimes caused by flies or dust landing in the drying. If you have heavy thick brush strokes in the paint then more rubbing down will be required and more undercoating as you'll be removing undercoat by having to rub down.

 

I would always consider 2 undercoats a minimum. I always thin the last undercoat if I want to achieve a nice flat finish Prior to Gloss,

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First off, painting in this weather is madness, you need to be in full shade at least, and even then I would think twice.

 

You only need to rub down undercoat if it's needed and not been left too long. Assuming a fairly smooth coat has been applied, the most rubbing down needed is a denib, thats a cursory flick over with something like 300 grit to remove small marks/flecks sometimes caused by flies or dust landing in the drying. If you have heavy thick brush strokes in the paint then more rubbing down will be required and more undercoating as you'll be removing undercoat by having to rub down.

 

I would always consider 2 undercoats a minimum. I always thin the last undercoat if I want to achieve a nice flat finish Prior to Gloss,

Many thanks for that, I'll certainly be looking at a thinner 2nd coat!

I'm painting in the builder's shed, prior to it being pulled out onto the canal. It's a case of 'go for it now,' whilst it's undercover, or take a chance with the elements in a week or so's time, when it's being fired out, regardless! The boat's in 2-pack primer, so I wouldn't fancy catching a long wet spell?

It is definitely too hot to paint, I've given myself a bit of extra sanding I could've done without!

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  • 2 weeks later...

Can I ask some advice re the final coat of gloss?

 

- bearing in mind the paint is gloss, would it be standard practice to very lightly flat off the paint to remove any micro particles/insects, etc. and then T-Cut it?

 

- also, if I have a run or brush blemish of any kind in the final coat, would I be looking at painting the whole panel again, or would it be acceptable to 'tease' it out with some wet & dry and t-cut it? Obviously the brush lines will be totally flattened off in a small patch, would this stand out a mile when viewing from the side following t-cutting?

 

- and finally, IF flatting and cutting is acceptable for final coat, what grade would you use at this stage?

 

Many thanks in advance, once again!

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This is where very fine wet & dry paper comes into play. It;s a very skilled and delicate task though as it's very easy to rub through the gloss with either with wet & Dry rubbing compound and even T-Cut, It would be almost impossible to do this with just one coat of Gloss applied. Much better attempted when 2 or preferably 3 coats of gloss has been applied.

 

We used to rub down Sprayed cellulose paint with 1000 grit W&D then use a fine polishing compound on a polishing machine to get a sheen and T Cut to finish off. The paint would be left a few days prior to this though. With gloss paints used on boats you would need to leave paint for quite some time to harden of enough to do this.

 

Personally If just trying to put right the odd blemish it's worth a try I suppose, but if there's a lot to put right though just flat again with 300 to 360 W&D and re coat, one coat of gloss really isn't sufficient enough on a boat anyway, minimum 2 coats.

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Can I ask some advice re the final coat of gloss?

 

- bearing in mind the paint is gloss, would it be standard practice to very lightly flat off the paint to remove any micro particles/insects, etc. and then T-Cut it?

 

- also, if I have a run or brush blemish of any kind in the final coat, would I be looking at painting the whole panel again, or would it be acceptable to 'tease' it out with some wet & dry and t-cut it? Obviously the brush lines will be totally flattened off in a small patch, would this stand out a mile when viewing from the side following t-cutting?

 

- and finally, IF flatting and cutting is acceptable for final coat, what grade would you use at this stage?

 

Many thanks in advance, once again!

 

It is not standard practice to consider cutting back as a way to improve a paint finish. Spend more time improving prep work and cleaning the surface prior to painting. Painting can only be improved with practice.

 

Too much white spirits and thinning will kill the gloss.

 

Runs - Hopefully, never any in the last coat. Runs don't cure properly, so, you have to deal with them as they happen, ideally. (oil based paint). If they are in what will be an intermediate coat, and you missed them while painting, take the run down with what ever you can, usually skimming back with a sharp blade down to the level of the surrounding paint. It must have time to cure. Skimming it back will not work just prior to the next coat, it will cause problems painting over it as the exposed paint will be soft underneath the skin.

 

Alot of runs are caused at the edge of window frames. Too much paint around the top and sides and bottom. Portholes can be very prone as all the excess paint follows the frame all around to the bottom with gravity. The top of the cabin side edge needs attention. laying off (tipping) strokes can leave a reservoir of paint at the top edge that runs down when your back is turned. It can be tricky gauging the amount of paint a particular area can hold.

 

The tip of the brush picks up paint as you're laying off. Keep an eye on it and remove any excess as you go along. You need enough paint in the brush to hold the hairs together. Too dry and it splits the hairs up, too much and you can leave more paint in some areas than you want (around windows, etc.).

Edited by Higgs
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Thanks, gents! I did paint a reasonable panel today :)...my biggest problem is a vertical row of six rivets every 18 inches along the side panels, it really keeps you on your toes, especially this weather!

My plan is 3 coats, but I was just wondering what the options were for any 'inconsistencies' in the last and dying hours of the final third coat?

If I shaved a tiny bleed under a rivet, I'd still need to lightly sand and T-Cut that spot, presumably...I'm taking it that this can OCCASIONALLY get you through, without leaving a nasty shiny flat spot in the panel?

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