Transferring white on dark shirt is tricky. The end result is a function of fabric and ink density. If white is a major color of your logo, your best bet is to lay a plain white in bottom first, then your logo. Of course it will be more expensive doing this. To get around you will either avoid using dark shirt, or avoid using light color on your logo.
The paper may be letting some of the fabric show through. Avery (and other off the shelf papers) tend to do this more than some of the "professional" brands. Usually it has to do with the thickness of the vinyl that you are printing onto. When I was first starting out I used an off the shelf on some pink shirts and ended up with a light pink background where it was supposed to be white.
If you haven't purchased yet, alot of folks here prefer the JetWear from tshirtsupplies.com. That is the same paper as Alpha Gold from Alpha Supply.
I like Ironall for Darks, I get it at New Milford Photo. Not that many other folks seem to like it lately. They say the surface is bumpy and they like a smooth surface, but I like it because it is thin, pretty soft feeling, and stretchy.
I started with Avery paper. I moved on to Ironall Dark. I can tell you a tip, any dark paper that gets too hot for too long will lose it's opaque quality and become translucent, nearly see thru.
Cutting back on the length of time the paper gets hit with heat, or reducing the heat, will help it not become translucent. When I hit the stage when the transfer becomes translucent, I chuck it aside and say I "nuked" another one.
Now, there is a trick I learned so I won't have to do that anymore. Maybe this will work for you. Take a plain piece of white paper (like photocopy paper) and put it over the image area and re-heat it. When it is good and hot, peel the plain white paper up, and the transfer paper will/should peel off the shirt for you. It may take a few tries to get it all off, but this is what they tell me around here. I hope I never have to use it, I'd like to try to avoid nuking anymore shirts, lol, but it's tucked under my hat. Hope it helps you.
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