I have not tried the Spray max 2K (meaning 2-part or a catalyzed resin so WEAR PROTECTION!) but did try a similar product from Upol that was similarly expensive and had the same sort of internal "capsule" of catalyzer that had to be broken and mixed inside of the aerosol can. With all that I was unimpressed with the results, but YRMV.
For my money I have had the best results with the strangely named clear ENAMEL from Rustoleum, no idea what makes this an enamel but the coating has been very reliably smooth and so far caused no reactions (lifting or crinkly "peel") on color coats (which have varied in brand and resin type.). Rustoleum seems to be replacing this with a '2X Ultra Coat' 'Painter's Touch' formula that is also good but not as smooth in the final dried finish as the older can, or so I believe. Again the name is strange since there's no "primer" in this formula and the "2X extra coverage" they promise is moot with clear since there's no pigment. They need a new marketing dept.

Not familiar with the Humbrol product but since it's water-based must be acrylic (I'd guess)and likewise would probably tend to be non-reactive with most dry/cured color coats as well as vinyl decals (stickers, transfers). One option I personally have not tried (but others may have) is commonly available in the US "Future" brand liquid "floor finish" from SCJohnson, I'm told this can be very smooth when sprayed and AFAIK is also acrylic liquid.
Do a trial application first!