I have worked with epoxy for many years. I have had my share of problems in the past, but the last 15 years I thought I had it figured out. Yesterday proved that I do not.
I have been working on my sons aluminum boat for a week now. I prepped the boat with 80 grit 3M sanding supplies and then coated it with the high strength Raka HP 900 and 631 hardener. No problem and a near perfect job with a few runs to sand out. After 10 hours I started applying the same mixture of 5 oz resin to 1 oz hardener into which 2 tablespoons of powdered graphite was added. I was heating the 6 oz mixture for 15 seconds in the microwave to make it flow out well and rolling it on with a 6" adhesive roller. When getting nearly complete I figured a double batch would finish the job and could be poured out on the broad area near the stern so it wouldn't kick off too quick. I heated the 12 oz mix for 30 seconds and poured it on the hull near the transom. I started rolling it out and it worked perfectly at first, then the roller started to smoke and what hadn't been spread was boiling on the hull. It affected an area about 3 ft long and one foot wide or 3 square feet total. I removed the worst of it with a scraper while it was soft (and hot). This morning I started to sand with the 80 and got nowhere. I switched to a 3 M stripping tool. (they look like a sponge grinding wheel) I have most of the worst ground off including the base. I will re-sand prime with pure epoxy and then re-coat with epoxy graphite. I figure the mess up cost me 10 hours labor and loss of the satisfaction of a near perfect job.
The moral of this story is if your system is working fine, don't get lazy and try to save a little time. Also microwaves may not heat on a uniform linear scale.