It needs a primer, otherwise the varnish will fall off. The acrylic just hardens and sits on top of the plastic.
It works as a temporary solution, and you'd have to regularly varnish it.
If subject to flexural forces (as if you were to drop it), the varnish will peel off. It will resist well compressive forces.
If it's for making artisan keycaps, something cool to do would be to use a crystal clear Polyurethane resin mixed with an oil based PU lacquer, on top of the cast. It will bond chemically, if used within the curing time (full cure is not in minutes, but hours)
Otherwise you'd need to sand them to promote adhesion and mechanically adhere to it. Sealers and glue work well on porous surfaces (paper, wood) because they drip in between it's pores and mechanically interlock in the outer layer, so it acts as a composite material. With non porous (ceramics, metals, plastics) surfaces it's trickier, and advisable a mix of mechanical adhesion and chemical, so sanding it is needed to grip to the base object, and the glue must be designed to adhere to the thing you're trying to stick to it, basically.
IF you are painting keycaps the pipeline is:
- (1) Base keycap prepared, deoiled, sanded.
- (2) Primer
- (3) Paint
- (4) primer* (you could skip this step if the varnish is of the same base that the paint)
- (5) varnish
Also, the solvent component in the plastic primers (around 20%) is most of the time too strong for keycaps and they start to melt quickly. I won't use spray based primers, or if needed a workaround is to spray on a sheet of paper or a tile, wait for it to evaporate a little, and then drip a clean non synthetic brush into it, and apply a thin layer over the keycap. That's enough.