Removing gelcoat on topsides

[RobbieH]

I have a 1970s vintage GRP boat with dark blue gelcoat. During the last 10 years it has been painted with 2-pack poly.

The topsides are covered (all over) with a huge number of tiny blisters/pimples. I had originally assumed these were a problem at the paint/gelcoat interface but on investigating I find they are in the gelcoat itself.

If I just sand through the paint to the gelcoat (knocking the tops of the blisters)blistrs do reappear after several months. If I sand deeper into the gelcoat tiny cavities appear and other blisters sometimes appear after several months - I am assuming these blisters are deeper in the gelcoat and sanding makes the layer above them thin (and weak) enough to let them "bulge". If I sand "through" the gelcoat to see the clear laminating resin underneath there are no cavities and the blisters do not reappear.

Based on this my intention is to sand off the gelcoat almost down to the laminating resin, prime, undercoat and finish with a one-pack poly (e.g. TopLac)

The question is what is the best way to remove the gelcoat while retaining a fair surface to take paint?

My current thinking is to do it the hard way with a longboard starting with a brutal 40grit and then back down to 120 to give a surface for a (hi-build)epoxy primer.

Anyone with experiences, thoughts, advice to share?

 

[Avocet]

Yes, I've been doing something similar on our boat! The topsides gelcoat has failed (thousands of tiny cracks, rather than blisters) but a similar problem to you anyway.

First of all, do you have a plain-sided hull or does it have any fancy mouldings (e.g. a cavetta stripe or moulded-in rubbing strake) in it? IF it's the latter, it VERY significantly increases the amount of work!

Secondly, if your 1970s boat is anything like my 1970s boat, the gelcoat thickness will vary considerably. Mine was between about 0.25mm in the thinnest places, and mearly 1.0mm in the thickest! Typically, it was about 0.4mm.

Thirdly, how thick is your boat laid up? Ours is typically 1970s - about 12-20mm thick, so I wasn't bothered about digging into the laminate by 0.5mm here and there!

I think you have two options. You can remove and replace the gelcoat (apply the new gelcoat with brush and roller, then sand it back and finally, buff it to a shine), or you can replace it with a mixture of epoxy and microballoons. There are a few threads on it here. Epoxy sticks better and microballoons are easier to sand. It's also more waterproof. On the other hand, it WILL ned painting and it's brown, so scratches will show through. Obviously that's not a problem with pigmented gelcoat.

Sanding sounds quite tedious - even with 40-grit. One method is to use an angle grinder with a concrete disc in it. Another is to hire a gelcoat peeler. My solution (being a tightwad!) was to use a big router with a 32mm bit in it. You can set the depth of cut to whatever you want, and then just push the router along the hull. It leaves a "trench" at whatever depth you set it to. It's very fast and if you plug a vacuum cleaner into the router, it doesn't make much mess. You then mix up the epoxy and microballoons and use the surface of the gel on either side of the trench to give you your line. Just fill the trench again, wait for it to set, and sand it down, then do the next one. The biggest problem is the curvature of the hull. In the very curved bits, it might be best to go down to a 25mm router bit and even then you'll have a trench that's shallower at the outer edges than the centre.

 

[oldsaltoz]

I doubt there are many people who could work a 40 grit long board, even a half long board would take some pulling.

Your first problem will be the fact that the depth of the original gel goat will vary, and it sound like it all needs to come off or the problem will not go away.

Hulls of this era were overbuilt so a full mechanical peel may be the way to go.

Fairing the topsides after a full peel is not for the faint hearted, mixing large volumes or resin and closed cell balloons then applying it and using a batten that runs from top to bottom first along then down the hull is no simple task, even if done in sections.

You can use the strip cut and fill technique as Avocet describes above but with a hull stripper, taking out a coupe or more strips the width of the peeler and leaving a strip a few mm smaller that the peeler. Fill and fair these areas then peel the remaining gelcoat strips and repeat the fill and final fairing with a long board.

I use the back edge of an old straight backed hand saw, the tooth side is ground off for safety. This being a tapered and flexible straight edge makes it easy to ensure both top and bottom contact points are full closed and the curve is uniform.

Hope this helps.

Good luck and fair winds.

 

[sailorman]

I have a 1970s vintage GRP boat with dark blue gelcoat. During the last 10 years it has been painted with 2-pack poly.

The topsides are covered (all over) with a huge number of tiny blisters/pimples. I had originally assumed these were a problem at the paint/gelcoat interface but on investigating I find they are in the gelcoat itself.

If I just sand through the paint to the gelcoat (knocking the tops of the blisters)blistrs do reappear after several months. If I sand deeper into the gelcoat tiny cavities appear and other blisters sometimes appear after several months - I am assuming these blisters are deeper in the gelcoat and sanding makes the layer above them thin (and weak) enough to let them "bulge". If I sand "through" the gelcoat to see the clear laminating resin underneath there are no cavities and the blisters do not reappear.

Based on this my intention is to sand off the gelcoat almost down to the laminating resin, prime, undercoat and finish with a one-pack poly (e.g. TopLac)

The question is what is the best way to remove the gelcoat while retaining a fair surface to take paint?

My current thinking is to do it the hard way with a longboard starting with a brutal 40grit and then back down to 120 to give a surface for a (hi-build)epoxy primer.

Anyone with experiences, thoughts, advice to share?

there was a bad batch of international undercoat about 20 yrs ago.
we had problems with our previous boat Co32, i had her taken back to gelcoat ( they used an air driven orbital sander 60 grit ) resprayed with Awlgrip 2 pack G-Line poly NOT The Acrylic as its too soft

 

[RobbieH]

Thanks for the replies guys it sounds like the routing strips technique is the way to go to keep a decent line. On potential problem is that due to past history - worked on a build of a Spronk catamaran and then built my own Wharram - epoxy and me do not mix well. Brushed or sprayed gelcoat doesn't sound like it will stick as well (but I could be convinced if enough people tell me it will work). I might have to end up searching for something half decent in between epoxy and polyester (vinylester?) that will stay on the hull but not make me swell up like a balloon.