~Did you look at my link - black spray foam can give good instant results without any sealer.
G4 "clear" has a yellow tinge like virtually any polyurethane varnish but it is available in black or green, you can mix the two colours to get various shades.
G4 is not that expensive about £15 per litre, that should be easily enough for a 30'' backdrop.
Epoxy, is not so easy to work with but will give a completely clear finish or any colour you like with added pigment however it is quite expensive at about £25 per litre & it wont stretch out like G4 either!
Re the magnets - cunning idea!