I want to drill out one of the screw holes for my drain plug, as there is evidence of water intrusion. I wondered if I can just clean it out, fill it with epoxy, drill a pilot hole, and put the screw back in? Or do I need to tap threads?

Are you talking about a mast?

You'll need to tap it. I would also consider putting a neoprene (the green ones) o-ring under the head of the screw so you won't feel the need to tighten it too much...that o-ring will also help to prevent the screw from backing out with trailer vibration.

I don't have any real data to back it up but the green o-rings seem to resist UV better than the black ones.

I also wouldn't directly tap anything that needed real holding strength into a composite structure...but you're fine in this case with a drain plug.

Jake's over thinking this.

Let the hole dry out. Clean it out with a pipe cleaner wetted with acetone.

Use thickened epoxy (JB Weld works fine for this - hard to measure this small amount of WEST) to fill the hole. Use a toothpick as an applicator and make sure the hole is filled with no air bubbles. Put a piece of tape over the hole to keep the epoxy from oozing out.

Several beverages later, peel off the tape, drill a pilot hole for the screw (no tap unless it's a machine screw) and reassemble with a dab of silicone around the drain plug housing. Don't overtighten the screws.

Thanks Matt! I think I overtightened the last time, and that may have been part of the problem.

mixing little strands of glass or carbon in the epoxy helps it hold screws much better.

I finally gave up trying to refill with glass strands and other bits and installed a 1/2" piece of G10 rod cut to the thickness of the deck and tapped it. Works perfect.