When working with forms and sealers, you can not be without good solvent thinners for cleaning tools and applicators and thinning your sealers when necessary. FS Thinner – Use FS Thinner to thin-out Form Seal. If your wood form is very porous; it’s a good practice to thin the Form Seal (2-parts Form Seal to 1 part FS Thinner) with FS Thinner for the first coat; followed by a full coat. Also, FS Thinner is excellent for clean-up of your tools and brushes.031 Thinner – Is a multi-purpose lacquer thinner. This is an industrial quality solvent with several active ingredients.