by JonLee on Wed Oct 28, 2015 12:43 pm
The masking tape method is purely to help give a nice clean edge to a new layer of fiberglass. You can certainly do without it, just at the likely cost of additional sanding required.
I usually use a 1" blue masking tape for this. I like the blue because of the colour contrast. I am not aware of any reason any old masking tape would not work.
At a summary level, the process is:
1. Tape off the area to which you are applying fiberglass. The inside edge of the tape is where the 'glass will eventually be trimmed back to.
2. Lay out the 'glass and smooth it down as usual. Trim the 'glass back to around the outer edge of the tape. It doesn't have to be too precise, though for best result you'll want it to overlap onto the tape by at least a 1/2 inch. I usually trim it to the outer edge of the tape and try to err on leaving it long.
3. Wet out the fiberglass per the normal procedure. Wet out at least 1/2" onto the tape, but try not to wet out beyond the outer edge of the tape, or you'll get the joy of additional cleanup.
4. Wait 1-2 hours to allow the epoxy to set up some before starting the "trim" step. Do not wait any more than 4 hours because you'll start risking not ever being able to remove the tape. Epoxy will certainly adhere the 'glass to the tape and and even soak through the tape and bond it to the subsurface. Also, you *can* trim the excess and peel the tape right after wetting it out, but at greater risk of not getting a nice edge (leaving threads, pulling up the cloth and introducing air bubbles, etc.). Waiting an hour or two is best.
5. Use a razor knife (Xacto knife, utility knife, etc) to trim the newly-wetted out fiberglass along the inside edge of the tape. Applying the right pressure here is key. You need enough pressure to cut all the way through the new fiberglass layer, but not so much that you cut into the substrate (whether it is plywood or fiberglass or whatever). Since there were a few questions earlier in the thread, I'll say it again...the goal is only cut the new layer of fiberglass and avoid cutting into whatever is beneath it. It is probably a little easier to get the right pressure than it sounds, especially if let the epoxy set up some (see step 4).
6. Pull up the tape. The trimmed-off fiberglass should come off with the tape.
The tape plays two key roles in this process First, it give you a visual line to trim along. Second, it facilitates the removal of the excess fiberglass by providing some "structure" for the 'glass you are trimming/removing.