Tuesday, June 3, 2008
Epoxy is fun. It's like play-doh for grown-ups. I've used it in tiny batches before, but never in large amounts or with fillers. Fun stuff.
Mixing up the first batch.
Be sure to measure carefully. I have big bottles of resin and hardener that you measure using pumps. It's a 2 to 1 ratio so 2 pumps of resin to 1 pump of hardener. The first time you use the pumps, be sure to not count that first bit that comes out on the the first pump. You want 2 FULL pumps of resin to 1 FULL pump of hardener. Mixing epoxy is an exact process. Measure properly the first time and save yourself a lot of trouble later.
Once your epoxy is mixed, the clock starts ticking. Pot life and cure time depends on several factors including the type of epoxy you use (slow or fast curing), the temperature (the hotter it is the faster it cures) and also how the epoxy is contained after it is mixed. Epoxy is exothermic so the more surface area that is exposed to air, the longer the mix takes to cure. If you leave the epoxy in the mixing container, it will heat up very quickly and can not only become combustible, but will also cure too fast. To lengthen cure time, spread it out like butter. I like to use pieces of cardboard for mine.
Once you have it thoroughly mixed you normally want to wet out the area you'll be working with.
Then add filler to thicken it for filleting.
The amount and kind of filler depends on the application. You'll need a peanut butter consistency for overhead or vertical areas, but not so thick for making fillets, finishing or bonding things. I needed a mix for filling some gaps and making fillets, so I used microballoons and mixed them until I got a good mix that wasn't too soupy but still thick enough to hold a shape.
Making a fillet.
Because epoxy cures so quickly you normally only mix a small batch at a time. Once you're done, you have a few choices. The easiest is to continue with another batch right away, or you can wait until it gets to a rubberry consistency and then add more epoxy. As long as the epoxy you just laid hasn't cured it will still bond chemically with new epoxy. However, if you wait until the epoxy has cured completely (usually about 24 hours) then you will have to sand or grind to get the new epoxy to stick. Once epoxy has cured it loses the ability to bond chemically with fresh epoxy, so you have to provide a mechanical bond by sanding or grinding, thereby providing a way for the fresh epoxy to 'key' into the old epoxy.
Ha! I sound like I know what I'm talking about.