Help please: epoxy/sanding problem




I've been fiberglassing/epoxying portions of my 16LT over the past few weeks, tidying up/repairing areas where the hull has suffered deep abrasions and along the deck/chine seam from which I removed last year's rubrails (which I chose to remove for, largely,  aesthetic reasons). I applied the final epoxy coat yesterday evening and took the boat outside this morning to do the sanding.


To my horrir, when I began to sand the epoxy, in many places, began discoloring (light brown) and turned "gummy." Did I not allow enough cure time? Did I get my resin/hardener ration wrong? How do I proceed?

I'd greatll appreciate your counsel.

 Many thanks in advance,


8 replies:

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RE: Help please: epoxy/sanding problem

If your temps have been in the 90's the epxoy might be hard enough to sand. The temp's where I live were in the 40's last night and 50's today. Probably not enough time for the epoxy to cure. I applied some epoxy when the temp in the garage was in the 30's and it took 3 1/2 days to get past the gell stage.

RE: Help please: epoxy/sanding problem

Well. . .that must be my problem. I live in Massachusetts and (w/ the exception of this past Friday when it was in the high 60s) temps have been between 25- 45. Mind you, I applied the 3rd coat of epoxy inside (my home) yesterday evening, but  clearly should have waited a few days to take the boat outside to do the sanding.

Is it reasonably to assume (hope) that the uncured epoxy I gummed up in my premature sanding attempt this morning will harden and be more receptive to sanding by, say, Tueday?


Many thanks,


RE: Help please: epoxy/sanding problem

The good news is that you did not alter the chemistry by sanding...the other good news is that there is a tuesday in every

RE: Help please: epoxy/sanding problem

Hi John,

Your room temp is very low to expect the epoxy to cure any time soon.  Once the ply has been cold, it must be the same temp as the expected curing temp for the epoxy to start curing like you expect.  If you scuffed the epoxy when not set enough, let it cure fully, sand it clean, if not looking good, re-coat and wait until fully cured.  Sanding creates heat, and when epoxy is not fully cured, it melts/gums at a lower temp.  Most fully cured epoxy melts at about 140 F.

Leave it alone until you can make no mark with your fingernail.  It usually take about 5 days for epoxy to cure fully at a constant 72 F.  Higher temps will definitely speed up  the cure.  Be sure to wash off the epoxy with warm water to remove any amine blush, also.  MAS does not make much blush, but there is some, just not enough to make much difference.  West makes more than enough to need careful wiping.

I built a 30' sailboat in a cold shop, warm when I worked, but cold when I left as it had a wood stove.  Fairing filler took three weeks to cure when I could only heat the shop every few days.

Good luck,


RE: Help please: epoxy/sanding problem

Dirk, many thanks for your counsel; much appreciated. J

RE: Help please: epoxy/sanding problem

f you are machine sanding the friction involved my be heating enoth fpr the epoxy to get gummy. Dont sand in one are longer than a few seconds before moving on and sanding a new area while the hot spot cools.

This comes from model airplane experience and surfacing epoxy so dont know about boatbuilding stuff.

RE: Help please: epoxy/sanding problem

Make sure the sandpaper is sharp. Worn or gummed up sandpaper causes problems.

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