Woodland Scenics Latex Rubber 16 Oz C1204

this is a really great product and the price is compatible with other brands

i use it for masking, simply scoop out into a paint dish, soak your brush in warm soapy water
then dip into latex to thin before painting your mask.

use plastic wrap to cover unused latex and it will keep for more than a day.

Does very well what I want it to do as a layer over model wire tree trunks. Takes paint well also. Downside is the price, but it does a lot of trees in HO scale.