It's not really a glue, per se... it is thin, but what it will do is melt the acrylic and the trim, and they will bond. More like welding....
Good description. The plastic surfaces are welded together. The chemical is not glue but a solvent—the solvent that is used in the manufacture of acrylic, I believe. The difference between No.3 and No.4 is speed. No.3 is faster. No.4 is considered "moderately" fast.
I also agree with
kgirl that after cementing the trim cap with fast solvent, it should then be followed up with a bead of Weld-On No.16, which is much thicker and stronger. The thin stuff keeps the letter held together while the heavier viscosity No.16 cures.
The thin versions are easily applied with syringes carried by
sign suppliers for this purpose. I think I use a 29 gauge needle. For No.16 I use a polyurethane "kechup" bottle sold by autobody supply shops. But pour the Weld-On back into the jug or can when you're finished or the solvent may melt your bottle.
Weld-On is made by IPS Corporation.
Brad in Kansas City
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Videos showing different methods of trim capping:
The nail method.
Using tape.
Magnets.