Prep for paintwork on GRP

[Balbas]

I've a Westerly Longbow which was born 3 years before I was. I suspect she's been painted a different colour at some point - hull currently blue and upperworks white, but showing signs of white on the hull and beige on the upperworks.

She's looking rough, scabby paint on the upperworks and a few scrapes and knocks on the hull. She needs a repaint this winter whilst she's out of the water. A professional job isn't on the cards - I don't have the money. I'm thinking of going to a uniform white finish on the hull and upperworks, her lines will always date her, but I think a more modern colour scheme will help her look less frumpy.

Having never painted GRP before, what prep do I need to do - get the scabby loose paint off obviously, but how without damaging the gel coat, will I get away with a power sander etc? What 'gotcha's' are there which would be helpful for me to know in advance? Do I need to prime and then paint, or can I paint straight onto the gelcoat? Presumably I then need to fine sand and repaint (several times, getting finer with each sanding) to achieve a good finish?

Also there's at least one place where I think the gelcoat has been damaged - a scrape along the side which I assume was caused by coming alongside a jetty or similar. Can I fill this with a GRP filler and then continue as above or is there another process I need to go through first?

 

[lw395]

Some undercoat goes very well straight on gelcoat sanded with 600 grit.
If you are using a marine paint system, then the manufacturer will have info on their website.
If you are using ordinary Dulux or some other make you may need to experiment a little.
I think it's best to fill gelcoat with gelcoat. Ideally grind out all the bits of star crazing, but it's up to you how far you go.

If you are going to paint high wear areas like the cockpit, consider two-pack polyurethane for that bit. Or these days there are some tough paints around in the car trade?

 

[wallacebob]

Innumerable posts on here, vids on YouTube, paint suppliers info. Have a search. Sand, Prime/undercoat, two top coats. Don’t over think it.

 

[Balbas]

Some undercoat goes very well straight on gelcoat sanded with 600 grit.
If you are using a marine paint system, then the manufacturer will have info on their website.
If you are using ordinary Dulux or some other make you may need to experiment a little.
I think it's best to fill gelcoat with gelcoat. Ideally grind out all the bits of star crazing, but it's up to you how far you go.

If you are going to paint high wear areas like the cockpit, consider two-pack polyurethane for that bit. Or these days there are some tough paints around in the car trade?

It hadn't even occurred to me to use dulux and the like. Does that provide any sort of wear resistance and does it adhere to GRP? It's bound to be cheaper than anything the chandler sells!

Innumerable posts on here, vids on YouTube, paint suppliers info. Have a search. Sand, Prime/undercoat, two top coats. Don’t over think it.

Yeah, trouble is sometimes there's too much information. I've tinkered with cars and done DiY forever, so if there's no special technique for use with GRP then I should be tickety-boo. I guess the one thing I'm worried about is the risk of damaging the gel coat with a sanding machine - but there's a lot of boat to sand by hand!

 

[Old Harry]

Some undercoat goes very well straight on gelcoat sanded with 600 grit.
If you are using a marine paint system, then the manufacturer will have info on their website.
If you are using ordinary Dulux or some other make you may need to experiment a little.
I think it's best to fill gelcoat with gelcoat. Ideally grind out all the bits of star crazing, but it's up to you how far you go.

If you are going to paint high wear areas like the cockpit, consider two-pack polyurethane for that bit. Or these days there are some tough paints around in the car trade?

I would use Hempel 2 pack to fill & fair + grind out any crazing

 

[scruff]

One useful thing to note - I used Hempel Multicoat on the deck of my Wayfarer last year. It promises to be undercoat and hard wearing top coat in one tin.

In real life it is softer and more wear resistant that water based emulsion and looks crap after 18 months. Don't go anywhere near it.

 

[Spirit (of Glenans)]

You seem to infer that the Topsides (hull) have been previously painted dark blue, in this case it is simply a matter of filling the scratches and dings with gelcoat filler and sanding down. Use 120-grit aluminium oxide paper to do the heavy work on the repairs, followed by 240, followed by 360. You can use an electric sander, but it can be difficult to control when doing the filled in areas, resulting in going right through the existing paint.
Next I would apply a coat of undercoat and sand this with 240 and then 360. Follow this with a coat of a 50-50 blend of undercoat and gloss, sanded back with 360, then finish with a coat of gloss.
With regard to older, lower value boats, the jury is out as to the merits of "marine" paint at a premium price as against tough domestic polyeurethanes. I went with Sandtex 10-Year Exterior Gloss.(No connection), ad supplemented it with about 8-10% Owatrol Oil to make it a bit more workable in the high temperatures we had this summer. Bringing along a sheet of glassor perspex for testing is considered advisable butI was able to test its consistency on an old perspex display case that was hiding behind one of the mobo's.
The paint was applied using the rolling and tipping technique.

 

[lw395]

In my view, a lot of marine paint is more flexible than say Dulux gloss.
That's good if you are painting a wooden boat.
Less good on GRP.
I did a GRP boat in Dulux some years ago, it still looked OK when I saw it some years later. 10 years on it had been painted again.

Personally I'm not great with a paintbrush. I would suggest it's a good idea to use whetever paint (and brush/pad/roller) you find easy to a get a good finish with in the circumstances.

On a higher value boat, I'd consider doing the prep myself then getting a more skilled person to paint.
For a smaller boat, I'd consider whisking it away to a spray shop.

 

[coopec]

I used a two part polyurethane paint and "rolled and tipped" which is a technique used by most professionals these days .Rolling and tipping is supposed to get a better result than spray. (I think it would be illegal to spray paint in the open these days)

Even though we painted before the sun got high in the sky and in the shade the paint was drying when it hit the surface and as a result it was a disaster,

When I was at the paint manufacturer outlet I just happened to mention our experience. His jaw dropped and he said " But you told me you were spraying! If we knew you were rolling and tipping we would have sold you a slow acting hardener. We have three, one for spray, a slow and very slow "

 

[Balbas]

I think it's best to fill gelcoat with gelcoat. Ideally grind out all the bits of star crazing, but it's up to you how far you go.

I would use Hempel 2 pack to fill & fair + grind out any crazing

You seem to infer that the Topsides (hull) have been previously painted dark blue, in this case it is simply a matter of filling the scratches and dings with gelcoat filler and sanding down. Use 120-grit aluminium oxide paper to do the heavy work on the repairs, followed by 240, followed by 360. You can use an electric sander, but it can be difficult to control when doing the filled in areas, resulting in going right through the existing paint.
Next I would apply a coat of undercoat and sand this with 240 and then 360. Follow this with a coat of a 50-50 blend of undercoat and gloss, sanded back with 360, then finish with a coat of gloss.

Thanks all for your comments. I have had a lot to think about recently with respect to boat maintenance and planning my winter's activity, and this has helped.

I maybe oversold the 'badness' of my current paintjob. The hull looks pretty ropey up close, but I think (aside from one scratch along the side) that it's mainly fine. There is an area on the bow where it looks as though a fender has rubbed the paintwork a bit, but generally speaking I don't think there's any gelcoat damage.

Everything from the toe rail upwards is scabby as hell, but *not* due to crazing or damage, simply the paint is coming off. I *think* it wasn't prepared properly, was painted in one coat and then after the boat sat on the hard for a few years and turned green, the previous owners pressure washed her in an attempt to tidy her up for sale (and made her look as though she had leprosy instead).

I've not seen any crazing or cracking anywhere on the boat (apart from one corner of the engine compartment hatch which is slightly crazed where a bolt has been overtightened).

So, assuming no gelcoat damage, the general consensus seems to be:-

• Sand old flaky paint off / key the hull
• Undercoat
• Sand back
• undercoat again
• sand back again
• topcoat (Hull 2 pack, above the toe rail Dulux will probably be OK)
• flat back and repeat using ever finer grades of sandpaper and a 'rolling and tipping' method of painting
• Have a beer whilst admiring my reflection

Is that basically correct?

 

[Spirit (of Glenans)]

Not just Dulux Gloss. You should use Exterior gloss, perferably polyeurethane. There is a Dulux paint marketed for front doors that is claimed to last 10 years.

Personally I would recommend Sandtex 10-Year Exterior Gloss (B&Q). This was what we used on a fleet of sailing school boats three years ago, and last Saturday a group of us went for a sail on two of them (Heir Island to North Harbour, Cape Clear and back!), and I can report that the paint job on the entire fleet is standing up remarkably well. Unfortunately I have no close up pics of the exterior of the noats

 

[Keith 66]

One useful thing to note - I used Hempel Multicoat on the deck of my Wayfarer last year. It promises to be undercoat and hard wearing top coat in one tin.

In real life it is softer and more wear resistant that water based emulsion and looks crap after 18 months. Don't go anywhere near it.

Awful stuff, Its soft & wears off in no time. Thats even with the boat sitting still & unused out of the water. Wont ever use the stuff again.

 

[steve yates]

So can this polyurethane exterior gloss be used on the hull?

 

[MikeBz]

If you're not going to remove all of the existing paint then that might affect your paint choice. 2-pack over 1-pot is generally not recommended, although you can sometimes get away with it if the existing paint is very old. If you are thinking of doing that then I'd suggest experimenting on a small area first (or not doing it...).

 

[Minchsailor]

If you're not going to remove all of the existing paint then that might affect your paint choice. 2-pack over 1-pot is generally not recommended, although you can sometimes get away with it if the existing paint is very old. If you are thinking of doing that then I'd suggest experimenting on a small area first (or not doing it...).

I go along with that. The more 'technical' the paint is, the more perfect the substrate has to be (and the more perfect the ambient conditions, such as temperature and humidity). On an older hull, where the history is not known, I would use a traditional paint; certainly nothing more exotic than single pack polyurethane.

 

[ronmarson]

Get yourself a Random Orbital Sander.
Sand to whatever finish you want. Don't be too fussy.
Then Sandtex, roll and tip off.
It's an old boat, not a work of art.
The first time you mess up coming alongside you will be glad you didn't spend too much time painting it.

 

[Humblebee]

Good point Ron, and one reason I'll stick with my old boat. Less to worry about!

 

[Humblebee]

Not just Dulux Gloss. You should use Exterior gloss, perferably polyeurethane. There is a Dulux paint marketed for front doors that is claimed to last 10 years.

Personally I would recommend Sandtex 10-Year Exterior Gloss (B&Q). This was what we used on a fleet of sailing school boats three years ago, and last Saturday a group of us went for a sail on two of them (Heir Island to North Harbour, Cape Clear and back!), and I can report that the paint job on the entire fleet is standing up remarkably well. Unfortunately I have no close up pics of the exterior of the noats

That's interesting, thanks. If paint stands up to sailing school usage it should be OK for the rest of us.
From memory, Sandtex is an upmarket version of Dulux, just as Sadolin is the better version of Crown Or the other way round perhaps?) Anyway, while specialist marine paints do seem expensive I also think spending a bit more on paint than just the usual DIY brands is worthwhile. Sadolin, Sandtex, Zinsser and others, even the trade versions of commoner brands, are worth the extra.
Looks like my own Longbow will get a repaint in the spring with quality domestic paint rather than marine. As someone else commented, it's a boat, not a work of art (or not the ceiling of The Sistine Chapel as I sometimes jest when doing paint work at home or for others!)

 

[Balbas]

If you're not going to remove all of the existing paint then that might affect your paint choice. 2-pack over 1-pot is generally not recommended, although you can sometimes get away with it if the existing paint is very old. If you are thinking of doing that then I'd suggest experimenting on a small area first (or not doing it...).

I go along with that. The more 'technical' the paint is, the more perfect the substrate has to be (and the more perfect the ambient conditions, such as temperature and humidity). On an older hull, where the history is not known, I would use a traditional paint; certainly nothing more exotic than single pack polyurethane.

Get yourself a Random Orbital Sander.
Sand to whatever finish you want. Don't be too fussy.
Then Sandtex, roll and tip off.
It's an old boat, not a work of art.
The first time you mess up coming alongside you will be glad you didn't spend too much time painting it.

Lots of exceptionally good points, thank all. In essence if it looks decent enough from 10ft away I'll be happy!

 

[wallacebob]

Did I say don’t over-think it? It’s an old boat, it’s your old boat. If you were charging someone for the job, maybe you’d worry. Otherwise, do it or don’t. It won’t sail any better, or have any increase in value. Not being rude, just practical. No need to reply, this isn’t a conversation.