How to thinn paint???

Well what i would do is to get the humbrol thinner and thin the paint 50% 50%, i mean 50% paint and 50% thinner. and if you are painting with a brush try to thin the paint a little bit more, i dont know if the humbrol paint is an enmel or acrylic but if you cant get the humbrol thinner and the paint is acrylic then i would use alcohol or if is an enmel i would make some experiments and see which thinner is appropriate for the paint that you are using, hope this helps.

Good luck
my best regards
Hector Reymundo