Follow the manufacturer's instructions precisely and don't experiment with catalyst ratios, at least not on a project you care about. Most conversion varnishes have a short recoat window. Failure to stay within the allotted time span may lead to poor adhesion between one coat and the next. The solvent-resistant properties of conversion varnish make it difficult to repair, once dinged or scratched, and almost impossible to remove with a stripper, should the need arise. The finish emits formaldehyde vapors, a cancer-causing agent.
Some brands emit the toxic fumes at several thousand times the established threshold value.
To apply this finish safely, you need a respirator rated for formaldehyde emissions and plenty of ventilation. The formaldehyde levels of some brands are high enough that the smell lingers in the dry finish for 24 hours or so, which is not good. Conversion varnish that gives off a low level of formaldehyde vapors is available, and it's worth searching out. You can request Material Safety Data Sheets from the finish manufacturers to compare formaldehyde levels of different brands. Even with all of these drawbacks, conversion varnish is still the first choice for many custom cabinet shops.