Three solvents of different evaporation rates are used: fast evaporating Dowanol PM propylene glycol monomethyl ether (1-methoxy-2-propanol) with an evaporation rate comparable with xylene, slower evaporating Dowanol PnP propylene glycol n-propyl ether (1-propoxy-2-propanol) with an evaporation rate comparable with white spirit or Shellsol 100 and very slowly evaporating Dowanol DPM dipropylene glycol monomethyl ether (1-(2-methoxyisopropoxy)-2-propanol) which dries as slowly as Shellsol D60.
The choice of gel or mixture of gels with different drying times depends on the scale and complexity of the retouching. As thinners one can use the solvents originally used in the gels (Methyl Proxitol from Shell is an equivalent of Dowanol PM, Methyl Diproxitol is an equivalent of Dowanol DPM) and Industrial Methylated Spirit. The addition of IMS increases the flow of the paint. The use of Shellsol 100 and xylene is not recommended.
All B72 Retouching Gels allow painting wet-in-wet, providing the same effects as those achieved by painting with artist colours or industrial paints.
Fast drying gels are most suitable for painting rich textures, for underpainting and for working on a small scale.
Slow drying gels are particularly suitable for painting wet-in-wet and for painting and glazing over large areas.
Very slow drying gel with Dowanol DPM has been especially developed for the painting of large scale reconstructions and for the glazing of large surfaces. It is also used as a drying retarder for mixing with other gels to facilitate wet-in-wet painting over a prolonged period of time. Gel with Dowanol DPM takes a very long time to dry and so gives the feel of working with oil paint.
B72 Retouching Gels vary in concentration of Paraloid B72 and their hardness/softness. By selecting the right gel or gels mixture the conservator-restorer can adjust the rheology and handling properties of the retouching medium to mimic properties of the original medium used by the artist.
Fast - Hard 25 Si, Fast - Heavy 20 and Slow - Heavy 20 gels are excellent for painting impastos and for painting with a spatula or palette knife. Fast - Hard 25 Si is suitable for painting extremely high impastos. Si included in the name of the gel indicates an addition of fumed silica in its composition, which provides an extra hardness to the gel resulting in rich, crisp, well-defined textures.
Fast - Hard 25 Si, Fast - Heavy 20 and Slow - Heavy 20 gels are also suitable for texturing fillings prior to retouching and for painting transparent textures on the top of the paint layer.
Fast - Light 10, Slow - Light 10, Very Slow - Light 10 gels are the ideal media for glazing.
Heavy 20 and Light 10 gels can provide results achieved by the artist using megilp.
Fast - Light 10 Si gel contains silica as a matting agent and was especially developed for retouching absorbent surfaces and absorbent, dry, matt paint such as gouache.
Fast - Light 10 Si and Fast - Hard 25 Si are especially suitable for underpainting. Their content of silica makes them more resistant to dissolving during further retouching on their surface.
Fast - Very Light 7.5 gel is used for applying extremely dry glazes that imitate dirt on painted reconstructions and is also used as an additive to a standard solution of Paraloid B72 in order to increase its viscosity without increasing its concentration.
The Gels are designed to be used in combination with solutions of resins where the required effect cannot be achieved by the use of a simple resin solution alone.