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I started fiberglassing the hull bottom Saturday at 1:00 a.m. to take advantage of a slight cool spell and falling temperatures. The hull-bottom 'glassing turned out to be quite an ordeal, and took me over four hours. After a great deal of trouble trying to align the cloth at the keel overlap (which only involved flipping the already laid-out cloth back over), I realized there was no way I would be able to align 16 foot lengths of cloth without help on a near-vertical surface, with it sticking constantly to the already-glassed bottom,etc. 5:30 in the morning was not a good time to be running around the neighborhood trying to get help with a difficult and messy job which none of the folks would have a clue about. Besides, the first part of my hull-bottom work had already reached tack, and I needed to do the second coat right away--which only took about 30 minutes.

So the decision was made that I would have to wait until the bottom was cured enough to sand the parts where the side-panel cloth will overlap. I will not get as good bonding, but I saw no way around it.

Finally, my question: I'm thinking of carefully sanding the parts of the hull where the side-panel cloth will overlap with 80-grit rather than the usual 120-grit. Granted this will be faster and easier, but that's not my reason. My thinking is the courser grit will give me better tooth for the epoxy to adhere to. Any thoughts?

Dana,
Are you using MAS epoxy? If so my understanding is this epoxy dose not create an amine blush (waxy surface) which means no sanding or cleaning between coats like the West System or System Three epoxy. Time is not an issue with MAS.
I'm not the expert but this is the reason I went with MAS vs. West System.
JUST FYI.
Jeff

Doing this by yourself is clumsy. I ended up wetting out the top part (the hull bottom and keel) on both sides, then sort of draped the side panel glass along that edge and pushed it into the wetted out glass enough so that it would stick. The glass isn't that heavy and it would end up sticking as I moved from one end to the other. Then I had to fiddle with it to get everything laid out.

I can't see that you are going to have a problem with adherance. As long as you don't have any blush, the epoxy will be fine. I did not use MAS, but I always make it a habit to wash the epoxy with water/soap and dry fully, then wipe with ethanol. I never had problems with "fish eyes" or beading after that.

I use MAS. My understanding from their literature is that subsequent coats should be applied after the previous coat has acqured tack, but before "thin film set." The epoxy should feel sticky with, say, a nitrile glove, but not come off on the glove. After the epoxy has reached thin film set you can no longer get a chemical bond, and it should be sanded to get a good mechanical bond.

If you are correct that subsequent coats can be applied any time without sanding, that would be great, but that's not the way I understand it. For example, in their FAQ #11 they state, "If more than 12 hours passes between coats, do a light scuff sand." To me that means you need sanding to get tooth for bonding. I have found that a light scuff sand leaves a lot of shiny areas, so I do more sanding to try to get a uniform dull finish for better bonding. This seems to be what CLC says, and they stress sanding to a dull finish once the epoxy has cured. I can only assume they are talking about MAS epoxy since that is the epoxy they recommend.

The reason for my question is that if you do too much sanding you can start abrading the fiberglass fibers or even sand through to bare wood, neither of which would be a good thing. So I was trying to get some opinions from more experienced builders about whether I should take a chance with a coarser grit.

Here's my plan to get the cloth to lie properly: I will use masking tape to hold it in place on the hull panels. Once everything looks good and I have cut darts (if needed) I'll apply the epoxy within about an inch of the tape. That should hold it in place, and then I can cut the dry cloth next to the tape and remove it, then epoxy what is left after cutting. I will make no attempt to pull the tape off the edge of the cloth, which would undoubtedly cause a huge mess. Cutting it with scissors should be the best way to go.

I've been doing some thinking and reading about this, and I'm waiting for a reply from the tech guy at MAS (although they are usually very slow to respond, if at all). Here are my thoughts:

Scuff sanding should be done with hand-held sandpaper or Scotch-Brite pads so that your fingers will naturally push the sanding medium down into the low spots. My plan* is to go ahead and knock off the high spots and runs/sags with 80-grit in the RO sander, then come back and scuff everything up with 80-grit sandpaper hand-held (no sanding block). I've decided to use 80-grit instead of Scotch-Brite to get better tooth.

I've tried this on a small section and it went very quickly and easily.

Future plan*: Later, the final sanding after all three coats of epoxy are applied should get everything smoothed out very nicely. Then it's on to establishing the water line and so on. After that I'll be using high-build primers (Interlux 404 on bottom panels and 4280 on side panels and topsides) to get the hull super-smooth for painting.

*"The best-laid schemes o' mice an' men
Gang aft agley,
An' lea'e us nought but grief an' pain,
For promis'd joy!"
Robert Burns, 1785

I got a definitive answer from the tech at MAS Epoxies--wish I had asked this years ago.

Here is a copy of his statement:

"There is a window of opportunity where no prep work is needed between coats when using MAS Epoxy. The ideal time to over coat is after the previous coat of epoxy has been allowed to cure to a tacky stage (tacky enough to pull the hair off a cotton ball) and up to the point when you can no longer indent a finger nail into the coating. Once you are past that window we recommend scuff sanding before applying the next coat of epoxy. We typically recommend 220 grit for scuff sanding, but 80 should work too, especially if you are trying to remove runs and imperfections. Just sand enough to rough up the gloss before applying your next coat. Be sure to wipe away the sanding debris with a clean rag soaked in a light solvent like denatured alcohol. Allow the solvent to flash from the surface before applying any additional epoxy."

A very generous window until the temperature gets above 75 or so. This is an easy rule to understand and follow and it gives a clear definition. I asked him to add it to the FAQs. I would add that I think the scuff sanding should be done with a bare hand on sandpaper or sandpaper wrapped around a sanding sponge, or with a Scotch-Brite pad. That way you will scuff more of the low spots.

Thanks for clairifying this operation! I learn something new every day...

Jeff

Glad to finally get this straight myself. I still think you made the right decision to choose MAS over West System. It is much easier to deal with, little or no fumes (I have done some 'glassing inside house with no problems), and I think the window for re-coating is more generous--and no amine blush!