Neill A

You could "fill" the open underside of the tray first so that it has a known solid and level base first. Let that cure and then set it down on the hopefully flat and level floor with a thin bed of adhesive. At least then you know the underside of the tray (i.e. below where you stand) is properly and fully supported.

I am a very recent convert to the Johnstones range of water based glosses: their Aqua range. The undercoat is simply awesome and the gloss pretty damn good for a water based paint. It dries quick with no real smell but has just enough time in it to brush out. Although a short fake mohair roller works best. Trick is not too put on too much of either undercoat or gloss in any one go and, paradoxically, don't overbrush it/stretch it out. If you do it loses its shine. Existing painted wood, use rough paper, eg 40 grit to 60 grit. For keying surface of painted wood I use 60 to 80 grit. For flatting undercoat or between coats probably 120 grade. I wouldn't consider 140 rough at all. For painting on top of varnish or lacquer try BIN 123 by Zinser: awesome methylated spirits based primer/sealer Cheers Neill