For pressure, this makes sense. For HEAT on the other hand, not so sure, depends a bit on how many flights you are likely to do.
Consider this video of an Estes motor test at around 1:45, burnout of the C motor with ejection charge. For a second or two there is an actual tongue of flame lazily burning from the front of the motor. I have always been pleasantly surprised that even minimum diameter cardboard rocket tubes don’t catch fire from the ejection charge. Shock cords, including KEVLAR shock cords, can and do burn through SOMETIMES (although they don’t to my knowledge catch fire) when they are mounted on the motor mount, I suspect because during packing they sometimes fold directly in front of the ejection charge. But minimum diameter tubing rarely, (interestingly, with the old Quest black powder motors, often the motor casing got so hot I would see “browning” of the white paint on the OUTSIDE of the body tube around the CASING. I have personally seen charring of a standard body tube when I went LESS THAN MINIMUM diameter with tubing just forward of the motor casing, and I think it is because the heat is concentrated over a smaller surface area. Doubling the tube thickness will almost certainly DELAY burn through for more flights, but it won’t prevent it. COATING it with another substance, white or wood glue, epoxy (JB Weld is often mentioned for its heat resistant properties
https://en.m.wikipedia.org/wiki/J-B_Weld#J-B_Weld_epoxy) may help, again the question is less, does the COATING hold up to the heat, vs does the intact coating sufficiently PROTECT/INSULATE the underlying surface?
aluminum itself is flammable, but aside from powder or shavings, seems like sheet aluminum holds up pretty well to multiple short exposures.
looking at the inside of a used Estes black powder motor, I think the inner layer of the paper DOES get physically charred. Since it is single use, it’s irrelevant. While homemade motor techniques are restricted to a certain section of this forum, if indeed the internal paper layer was intact, I would think the logical cheapest way to make motors would be to recycle used casings (tube and nozzle already there, would simplify at least one or two steps of the process.). I am thinking the reason it isn’t done (and yes I am reaching here, I neither have nor want the experience of building my own motors, it’s not my gift), anyhooo, the reason it isn’t done is I suspect because the used casing don’t have sufficient integrity for re-use, at least in part BECAUSE the inner layer is cooked.
perhaps those with more experience building motors can set me straight on this?