RC, are you running these tubeless? If you are, and you don't see any cracks or other defects.......and these blisters appear to be filled with something, you might be seeing the effect of air having gotten through the plies from inside the casing and are now inflating small sections of the sidewall rubber.
I guess you could "prick" one and bubble check it with soap.
You might want to bubble check it with soap anyhow (without busting a blister) if it's tubeless and your tires have been getting low of air pressure.
JMHO

mowie, no cracks yet. I got tubes in them, the blistering not bubbling effect, kinda looks like burnt from heat. I ck the air press quite often and keep them between 10 to 12 psi.. Rarely have to add air unless there is an extreme temp change. I tried to clean one side and WD40 it made it look shinny, but no affect on the deterioration.

I sounds like the tires are just deteriorating. True they should last longer but environment can reak havok . My original tires lasted 12 years dry rot and umpteen holes did em in , replaced with 4 ply tires rode a little harder but peace of mind for less punctures. I heard of Armorall on tires but WD-40 ? I dont know what chemicals are in it to possibly break down the rubber, if you are like me which I wouldnt wish that on anyone, if they hold air runem season to season keep an eye on them replace when needed.

Doberman: WD-40 has Silicone additive which as a moisture dispersant also will preserve rubber . Have used as a freeze protection on Door Seals after washing to preserve and prevent sticking doors during colder weather . Armor All is probably the best for UV Protection . Both work very well for tar and road grime removal also on car finishes.

He's back!

Here is a link that might be useful: Farewell

The deterioration of both of your rear tires at the same time and area of the tire makes me believe it is a function of age. Ozone causes tire deterioration but I think it usually results in cracking. If you have a big air compressor with a large electric motor that runs a lot near the tires, Ozone could be a contributing factor.

Ozone is a major component of much air pollution. It is the reason why many rubber products "don't last as long as they used to". For example rubber bands.

It may or may not be the issue here, but it certainly could be.

Or maybe they just came from a bad batch.

When we sold tires it never failed a set of odd sizes would be stuffed in a corner . After a couple of years you wouldnt sell em the sales crew would finaly fish em out of the rack and write em off... we always got credit ,they got hard but never cracked but not as pliable. I feel tires are a lot better than yesteryear.

The tires on my '77 IH Cadet 80 are original. They look it too. Yrs ago, I dosed the the wheels as needed with fix a flat. It seems to have cured a slow leak, in the rear. If they aren't flat, I wouldn't worry about them.

WD40 is the wonder chemical can do about anything. It cleans and shines. Here is an example if I remember how to post a pic.{{gwi:356060}}

Wooa, that was way to big, try another. Brought this JD for 400 bucks with bad engine It's at least 11 years old?. Got engine off ebay and have $750 in it. has 356 hours now with about 45 on the new engine. ready for my snow blade.
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Nice!

Nice Unit RC ! Really great when you can refurbish a fine unit like this at a Fraction of the Original Cost . Are you going the chain or rear ballast route for traction while plowing ?

chains, may rig up some weights off the back. But my 200 pounds may be enough LOL.

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Again RC ..Nice Job Dude !