i am currently experimenting with M.Graham&Co. and Sennlier for a few of the "eventually-turns-hard-as-rubber-when-dried" tube colors. the honey in the binder (hydrophilic) may make them re-wet better for a longer period of time. 

witnessing what happened to some my friends' paint sets in the tropics made me think of trying out these brands -- at least for burnt umber, viridian, etc.!

jenny