Personally I wouldn't use any type of powder. They make sprays that are available that are designed specifically for slide out seals. Any type of powder will leave residue that will tend to clump and interfere with things when they get wet. Naturally if your RV is never out in the rain, or is never washed, then you do not have to worry about any powder getting wet. The spray products have worked well for me on my slides.
Have and look and let us know back here which one you're going with.
Don Ford 2004 Safari Trek 31SBD (F53/V10 20,500GVW) '09 HHR 2LT or '97 Aerostar MiniVan (Remco driveshaft disconnect) for Towed vehicles BlueOx Aventa II Towbar - ReadyBrake Inertia Brake System
2008 Newmar Mountain Aire 450 HP CUMMINS ISM ALLISON 4000 MH TRANSMISSION TOWING 2014 HONDA CRV With Blue Ox tow bar A man who carries a cat by the tail learns something he can learn in no other way. -Mark Twain
I saw that tip in a video by the RV geeks on YouTube. I have never tried it myself, I use the regular rv foaming stuff. Works fine for my seals. The rv is 13 years old and the seals are still very supple