It depends on the tape.

PVC/Electrical tape tends to dissolve.

Masking tape or packing tape should be fine as long as you don't mix the 'resin' so that it heats up (exothermic reaction) to the point where it melts the tape ( don't put too much hardener in ).

Most if not all polyesters and for that mind epoxies will set with out exposure to the air AFAIK.

A bit different. I mixed up some epoxy resin and put into a mould which had some making tape around the top. The epoxy reacted with the glue on the tape and made a load of foaming bubbles. In fear of an impending explosion I placed the whole lot in the middle of the garden. To my disappointment it fizzled out and was rather uneventful. The part of the epoxy that reacted with the tape was very weak and brittle.

I found this a good way of making a repair to something like the gunwale. Use a mate's undamaged laser to make a mould of the part you damaged. Slap some vaseline on the gunwale and place strips of newpaper on the vaseline (it will stick to it). Then build up by stricking more strips of news paper on with pritt stick. One you have built up to a reasonable thickness remove and place on a radiator for an hour or so. This will make the mould harden off. You then have a near perfect mould for which to place on top of yor repair to get the right shape.

You can use this technque to make mould of pretty much anything. I made epoxy mast chocks for the 300. Just remember to criss-cross the paper strips otherwise the mould will not have any strength in it.

Apparently the different resins harden in differnet ways...

Epoxy resin uses an oxidising hardner (exposure to the air makes no difference):-

Polyester resin (gelcoat) needs to be protected from the air, so I believe it uses a reducing hardner. (i.e. removes oxygen from the monomers).

That means that gelcoat put onto uncured epoxy will never set (I can confirm that!), and that either you exclude the air by taping (sellotape is good) over the gelcoat - or you can get gelcoat resin with a wax in it (this forms a skin on the repair and lets the gelcoat harden.)

If you put MW solution (basically paraffin wax in styrene) in your gelcoat, it is designed to act as an air barrier and allows the gelcoat to go off without tape. However, you won't get the smooth finish, of course. Plastic Padding gelcoat filler (the white stuff) already has this or something similar in to allow it to go off.

If you don't have the right colour Laser/RS etc colour, with lots of fiddling you can get pretty close using either small amounts of acrylic paint or powder paint to change the colour. I was surprised that gelcoat still set with acrylic paint in it when I tried a bit from a model set one of the kids had.