* The clogged expansion valves (old capillary tube type) that I've experienced were clogged when I cut corners, not by the sealant. As part of a 134a retrofit, I've never had that problem when I flushed the condenser and evaporator circuits first and then replaced the expansion valve and receiver/dryer.
* The original capillary tube expansion valves came with a screen insert (replacements I've used recently don't have the screen), backflushing will clear the debris. But backflushing is almost as tedious as replacing the exp valve.