The underglaze was painted on to bisque ware (has not be fired on). This is a problem. It has a high gum content and has sealed the surface so the porous body underneath is unable to pull water out to dry it quickly. During the slow dry the little absorption that is taking place is generating air bubbles from below and these are producing bare spots. The solution is to either make your own underglaze having a lower gum content or decorate ware in the dry or leather hard stage so the bisque fire will neutralize the gum.
A ceramic compound meant to be applied to green or bisque ware and covered using a transparent overglaze. There are good reasons to make your own underglazes if you are in production.
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