don't have to be old rubber to be porous. i just got new lawnmower tires.. right outta the smelly chinese packing box, and on the shiney painted rim.. the sidewalls will 'foam' soapy water :(
in any event. what i'd do?
lay a boot down in the tire casing.. boot can be a real boot or one you make from some stall matting.. etc.. then put in a tube and go. boot will absorb most tread punctures unless you run over something amazing...
no help on sidewall.. but then.. same with or without a boot. ANY time i have a constant leaker tire on a non auto vehicle.. it gets a tube... sure.. you got to break down a tire to tube it.. but saves the hassle of the constantly flat tire.
harbor freight sells a manual bead breaker tire changer deal fro about 50 bucks.
bolt it down to the concrete and go... handles up to 16" rims I think?? though I have made it handle 19's with some adapter plates I made.. worked fine...
Soundguy...seems you recommend using a boot or tube rather than putting sealant (Slime) into a leaker or using instant flat fix from a can. I have only used something like Slime once in my life, I needed to seal a pair of tubeless 13X5 bar tread tires on a snowblower...worked pretty well. Bead breaking and installing tubes is a huge PIB unless one has experience, really easy to pinch the tube with a tire iron and you automatically have a flat already. Last tube I installed was a wheelbarrow tube, not too hard but anything bigger would be a headache as the only tire irons I have are about 8 inches long.
imho.. larger tires are easier to break down than smaller.. and tube management is easier than with smaller.
ie.. mounting a 28" tire gives you more room than a 13" or 15"
i use a 50$ manual tire machine from harbor freight to help with small tires. large tires you just lay on the ground.
inflate the tube a bit to keep it out of the way.
a coule good tire irons are a good investment.. make good pry bars.
you get experience by doing something.. about the only way to gain it.. :) is to do it.
once you start doing them, AND have a sledge and some tire tools.. it's really not too bad..
I have used a sealer product before.. with mixed results. have had it work great.. have had it not work at all.