If you are ordering a wall decal, it should be a matte finish and the adhesive is not super strong (to allow you to take it off without wrecking your painted walls). Here is a picture of what can happen if you apply a thick coat of polyurethane over a wall decal sticker.
Polyurethane can cause the edges of the decal to curl. Now I am not saying you CANNOT seal your decal sticker.... we have had customers tell us they have done it without any problems.
Here is some things to consider:
If you have a Wall Decal (matte finish and not super-strong adhesive), use a VERY light coat of polyurethane. Mod Podge is another product that I have heard customers state they have used over their wall decal stickers. The polyurethane tends to curl the edges of a vinyl sticker and with the lighter adhesive, the strength of the adhesive can break down.... on a Wall Decal Sticker.
Another factor on the above board might have been the seams of the wood - this allowed the polyurethane to seep under the decal sticker just a bit and this reacted adversely with the adhesive on the decal sticker.
When our customer sent us this photo with "Help!" as the subject line, we suggested we get her another decal (we generally offer a mess-up decal replacement at half price). The second decal sticker would be made with the GLOSSY OUTDOOR grade material, though.
The glossy was not a factor seeing the polyurethane she added made the decal glossy-looking anyway. And our outdoor grade material has a MUCH stronger adhesive... once that sticker sticks to a spot, there is very little chance you have to pick it up and move it to a correct spot! So the stronger adhesive of our Glossy vinyl material was a much better fit for this project. The strength of the adhesive allowed for a strong application to the wood and therefore wouldn't curl the edges of the decal when a light coat of polyurethane was applied.
Generally a seal is not needed for a decal made with our Glossy outdoor grade material, but if you want to feel safer, go ahead and apply a coat over your decal, but, please, make it a LIGHT coat! The beanbag toss board picture below is an example of using a light sealant coat over top the decal stickers.
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