Here is my pic and full attribution. Unfortunately, someone cleaned this coin a little too roughly. You can see bare bronze in a few places - that didn't come through in the seller's pic. But I don't think it's anything serious. It will darken over time and look more natural. Theodosius I AE3, 17mm, 2.7g, 12h; Siscia mint fourth period: AD 378-383 Obv.: DN THEODO-SIVS PF AVG; Daidemed, draped, cuirassed bust right. Rev.: CONCOR-DIA AVGGG; Roma seated facing, head left, holding globe and reversed spear, her left leg bare // BSISC. Reference: RIC IX Siscia 27d (p.151) Scarce.
I'm not sure what you mean by "gum," but I basically use a paint bucket to remove most of the background the coin was photographed on, which is black anyway, then I use a paintbrush around the edges of the coin. It's not sophisticated but it works for me.