I don't know much about the Jeffco tank sealant for the inside of the cowl.  I have had good luck with the Imron that's on the inside of my cowls.  The weave was filled, with the inside cowl finish as smooth as the exterior of the plane.  With white on the inside, it is very easy to see oil drips, dirt, avgas streaks, etc.  And cleanup is a snap. 

 

I didn't want to use heat insulation blankets of any kind, as they are difficult to keep clean.  I do have some heat shields in places near my exhaust system to help reduce cowl heat.  

 

Of all the Lancair models, the 235 airframe is probably the most tightly cowled of all of them.  Shoehorn an O-320 in there and it gets even more crowded.  Thus, my cowls are really close in places near the engine (exhaust system, intake tubes, alternator, starter, etc.) with less than an inch clearance in some places.  The Imron has held up admirably well after 135 hours.  Thus far, there has been no paint discoloration on the outside of the cowls.

 

Gary Edwards

N21SN