No, really - blushing.
How can you tell?
I've been using epoxy.
Rather than lash frames, we've opted to bond the stringers to the frames with epoxy. In this case, I happened to have a left-over kit of System Three's general purpose epoxy resin. To create the adhesive, we have added fumed silica to the resin. (Note the dust mask that the student who was mixing the silica into the epoxy was wearing! An excellent idea as you don't want to inhale the stuff!) The System Three general purpose resin blushes a bit. By that, I mean that it has an amine blush that rises to the surface f the bond joint. This particular epoxy is amine based and the amine blush is a waxy coating that rises to the surface of the curing resin.
This amine blush can raise havoc with the curing of finishes and the bonding of successive coats of epoxy if you want to apply them. However, it is easy to remove this blush. There are two tried and true ways. One is with soap and water, the other is with household ammonia. As we intend to coat the frames we're building with varnish or polyurethane before skinning them, we want to remove the blush to prevent problems with curing that can include retarding the cure or fish eyes in the finish. You need to remove the blush before sanding so that you don't simply spread it around.
There are some resins that claim to be "blush free" - particularly clear-coat epoxies. I tend to treat this with a grain of salt and assume that any epoxy that I use may have some amine blush. It's a case of "better safe than sorry".
I'll tell you why I tend to be careful. The first strip-built boat that I glassed with a particular vendor's epoxy said that I should prepare the surface before putting on a protective layer of UV filtering varnish. I made the assumption that they meant sanding, and thoroughly wet-sanded the hull with water and fine-grit paper. I then applied a good quality spar varnish with UV filters. The next day, I went downstairs and touched the hull, leaving a finger print. I figured that it was a bit cool in the cellar and let it wait. I came home from work - same thing. I let it sit two more days - a third finger print.
After a call to the manufacturer's help line, I finally wound up removing the uncured varnish with solvents - a nasty, messy process. I then prepped the hull per their instructions (ammonia and water) and then used their recommended proprietary coating. I did have success, but learned a valuable lesson.
ALWAYS REMOVE THE BLUSH!