Some customers avoid the high priced mint stamps because they can't tell if the stamp is regummed or not. Here are four tips that may help.
First, you need a very powerful magnifying glass, 30X or more. Looking at the face of the stamp, scrutinize the holes for the perforations. Stamps with original gum show absolutely no traces of gum in the perforations. Stamps that are regummed may show tiny traces of gum in the edges of the holes. That's because stamps are gummed before they are perforated. Many times, the regumming process will leave tiny traces of gum in the edges of the perforation holes. Regumming techniques have improved over the years and some of the more recent work is very difficult to detect.
Second, look at the gum itself. Do you see brush strokes? Original gum stamps show no brush strokes because the gum was applied with a roller and not with a brush. Some amateur regumming jobs will show brush strokes on the gum side.
Third, compare your stamp to a lesser value stamp of the same series which you know is original gum. For example, the same gum was used for the $5 Columbian (Scott #245) as the 1¢ Columbian (Scott #230). Buying an original gum copy of Scott #230 isn't too expensive. You can use the gum on that stamp to compare it to higher value stamps. There are minor variances in gum used on stamps over the years. That's why it's important to use stamps from the same series for comparison.
Fourth, place your mint stamp on a flat surface, stamp side facing up. Does the stamp lie flat? Or does it have a large hump in the middle as the edges of the stamp tend to curl underneath? If so, there is a good chance that the stamp is regummed.
All mint stamps have a tendency to curl, especially if they have been exposed to moisture. With original gum, any curl is usually very minor and the stamp lay relatively flat. When a stamp is regummed, the gum shrinks as it dries. This causes the edges of the stamp to curl underneath towards the gum side of the stamp because the perforations are tiny, weak pieces of paper.
If you lay a stamp face up on a flat surface and you could drive a Mack truck underneath, it’s a good bet that it has been regummed.
These tips may help you identify some of the more obvious regummed stamps. When in doubt, consider a certificate from one of the expertizing agencies. But before spending money on a certificate, you may be able to weed out some of the regummed ones by yourself.