Hi Adrien fillings
Personally wouldn`t use salt. (But I cant say it doesn`t work ; I`ve not tried it)
I`ve always used Epoxy resin (Araldite; but not fast settin;one package will just do a board) and white plastic granules. You can buy granules from Gun sails (Think theirs are foam based) or make your own from filing any firm white plastic. (I`ve used both white ruler and white plastic blocks (used in furniture building); just file them with a medium file/course sand paper and allow fillings to drop on board. I have used a foam block held against a sanding machine. (Dont try that with plastic) (You need clark foam for that though;polystyrene doesn`t work)
Get board; and epoxy resin really warm before spreading . (Its a lot thinner and goes further)
I dont mix resin and powder.(The bits would float to top anyway) Roller resin onto board (mask out bits you dont want covering) and then "spray" with either plastic bits/foam bits. Leave alone for a day
Its not a hard job.