I've had a few Wing kits - no problem with the foam if you use clean and not over 150 grit sandpaper.

Had a bad experience with a Sureflight foam P-51. Foam rolled up a lot on the sandpaper causing grooves I had to spackle and sand over and over. Wound up in the dumpster.

For my money, Wing short kits are a good deal if you can get cheap balsa. Don't overpower them or they're REAL fast. Even put mechanical retracts in one and they worked fine.