vomit, i don't know how to thin etch, try mixing a little with water. if you really want to do it, look on the bottle and see if it lists the ingredients, also look for a MSDS document for it online, then you'll know what to use. you could use it like it is by smearing a rag or paper towel in it and using it that way, change gloves often. or put it in some container, like a thing with a brush on it used to put sauce on meat.
whiteox, i doubt it's because of the stuff in your marker, mix the garvey with something opaque, use a little acetone if the solvents are different.