Hairline cracks in deck gelcoat - ignore or fix before painting?

[CamWrenRoo]

Hi folks!
So we're preparing to paint our decks. 80's gelcoat and old paint jobs have degraded to a splotchy, chalky white mess. I've got a few bad repair jobs from previous owners as well as some screw holes from an old dodger that I will be filling, faring, sanding etc, and I intend to lightly sand and wash down the whole boat prior to priming and painting.
Now the 'spider' cracks... They are numerous and just about everywhere. Stanchions, old repairs, curved areas, windows... Everywhere. So, I'm wondering, should I suck it up and grind out every last one of them and then fair them along with the other spots I'll be fairing? Should I spend a small fortune on a non grinding alternative like MagicEzy (I've used it on a prior boat, and I know I'll need at least 5 tubes to do all the cracks I have!). Or, should I just ignore them? I don't really mind that they are there. And I think that a solid coat of primer and paint will probably hide them fairly well. I do not have a ton of time on my hands... Fyi - no signs of water damage, decks are firm and sound good,so I don't think I have any water getting into the coring.
If you were me, what would you do, and why?
Thank you!!!!!

[s/v Jedi]

Assuming these spider cracks are only in the gelcoat: sand away at it. When you sand half away, the cracks are only half as deep. When you see you got to solid gelcoat, stop and prep for primer. Use a good 2-component epoxy primer like Awlgrip 545. Now inspect cracks again: fill them with epoxy filler (I switched to TotalBoat) or if it looks bad, grind out a bit, add some fiberglass with epoxy if needed, then filler and sand again. When all looks good, second coat of 545. I reduce this rather hefty with brushing reducer to eliminate orange peel. Next follow up with the top coats.

[CamWrenRoo]

Originally Posted by s/v Jedi

Assuming these spider cracks are only in the gelcoat: sand away at it. When you sand half away, the cracks are only half as deep. When you see you got to solid gelcoat, stop and prep for primer. Use a good 2-component epoxy primer like Awlgrip 545. Now inspect cracks again: fill them with epoxy filler (I switched to TotalBoat) or if it looks bad, grind out a bit, add some fiberglass with epoxy if needed, then filler and sand again. When all looks good, second coat of 545. I reduce this rather hefty with brushing reducer to eliminate orange peel. Next follow up with the top coats.

Excellent, thank you so much. I like the total boat for fairing, too. I ground out and filled the cracks on my last boat, and it was awful. I just don't want to do it again! Sanding them down a bit and then priming sounds like a good compromise.

[barnakiel]

I used a soft (sanding) primer first and then sanded it flat with the gelcoat.


My gut filling is that one can also do without this step, unless the cracks are no longer hairline.



b.

[CamWrenRoo]

Originally Posted by barnakiel

I used a soft (sanding) primer first and then sanded it flat with the gelcoat.


My gut filling is that one can also do without this step, unless the cracks are no longer hairline.



b.

That's a great idea. Thank you!

[psk125]

The Awlgrip 545 primer, after sanding, filled in almost all the cracks and crazing on our deck. We spot filled a couple of gouges & dings with epoxygoop prior to sanding, but didn't grind out.

[s/v Jedi]

Awlgrip 545 is a high solids primer and has a stellar reputation. On that reputation alone I would opt for it rather than their high build primer.

To stabilize the cracks and gelcoat, thinning it beyond recommendation makes it penetrate better, but that also means you do not build up enough thickness so you need to follow it up with another coat thinned at their maximum recommendation and consider these two coats as just one coat. My 25year old gelcoat gets as solid as imaginable after that first thinned coat and the second fills the cracks rather well. I then use the TotalBoat epoxy fairing compound over all cracks, damaged spots etc. The color difference helps with sanding after that. The next coat of primer I try to get as close to finish quality to make the top coats easy. This means maximum brushing reducer and the hard foam rollers. These rollers only last so long before falling apart so I replace every 15 minutes.

When you don’t reduce the primer, you get orange peel so get to do more sanding. Sanding is easy though.

For top coat I use original Awlgrip but I have no direct experience with other paints myself so can’t compare it.

[Hardhead]

If they are just the run-of-the mill hairline cracks - I think you can leave them if you like. I've had hundreds of such cracks, on boats that have lasted 30 years them, and never an issue outside of cosmetic appearance. I don't actually think I've ever seen a fiberglass boat over 10 years old without them.

[GeneM]

I use Interlux Perfection and don't sand down or grind out hairline cracks. Do be sure to wash with degreaser before sanding; I use 409. When washing off note that there's no beading up of water. It should be a flat sheet as it flows off.

I wash then sand with 150. Then two coats of primer. Then sand again. That should cover hairline cracks.

Then I do two coats of Perfection, thinned as recommended. Be sure to lay it on thick. Be less worried about sags and runs than dry roller drags. Thick rolling will level out nicely
without tipping and give a shiny surface.

Use foam rollers. I use solvent resistant from Ace Hardware. But they only last for 4 to 6 ounces before dissolving.

[StoneCrab]

Andy from Boatworks Today discusses this in several videos. He takes a pencil grinder and grinds out the cracks and then fills in the grooves. If you don't address the cracks they will just telegraph through the paint.

https://www.boatworkstoday.com/?s=cracks

https://www.boatworkstoday.com/video...awlgrip-paint/

[s/v Jedi]

Originally Posted by StoneCrab

Andy from Boatworks Today discusses this in several videos. He takes a pencil grinder and grinds out the cracks and then fills in the grooves. If you don't address the cracks they will just telegraph through the paint.

https://www.boatworkstoday.com/?s=cracks

https://www.boatworkstoday.com/video...awlgrip-paint/

Yes, when there are cracks at stress points like corners, mechanical fasteners etc. that appear in otherwise healthy gelcoat. Such areas need to be reinforced with additional glass.

When there are hundreds of cracks then the gelcoat is at end of life and even the smallest amount of stress will crack it. This is where sanding away the gelcoat is the most direct method of getting rid of them. I’ve seen some video footage from Andy where he grinds out the cracks and fills them with a (sponsored) thickened epoxy. You must grind them out to get that epoxy in the crack. On the other hand, when you use 545, which is an epoxy as well, it can penetrate the cracks (and porous gelcoat) without grinding them out.

In reality, you have to decide which cracks can be sanded and the epoxy primer and filler will be all needed, to cracks where after sanding some more grinding and filling is needed, all the way to cracks that need even more grinding and fiberglass added before filling. To know what to do where is mostly a matter of experience I think. Sanding most gelcoat away is helpful for sure, especially in areas where way too much gelcoat was used.

[Captain Anthony]

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[CamWrenRoo]

This is great everyone, thank you so much