I would do something ridiculous.

I used to use quite a bit of wooden boat fairing compound that I think would work in this application. Comes in a pint or quart tub from Hamilton marine. Interlux used to make (perhaps still does) a plywood sealer that really did a miracle on weathered cracked ply.

dolfin glaze or similar, its made for this type of situation. filling deep grain with shellac could be considered ridiculous.

I've done some that seem to have this deep wood grain. Here is what I used from Evercoat:

GLAZE-IT® is a two-part, flowable, polyester finishing putty ideally suited for filling and skim coating body work, pinholes, grind marks, low spots and other surface imperfections. Use as a finish coat over body filler repair areas to fill minor defects, deep sand scratches and pinholes prior to applying primer surfacer to improve the efficiency of the primer coat. Can also be used to fill minor body damage up to 1/8” in depth.

Why is Bondo ridiculous? It seems to me you want to do more work than just Bondo it.

Shellac primer is good for small to really fine wood grain..i wouldn't suggest to build shellac...thin coat of bondo would be your best bet....

I have found that to be a fairly common issue on cab sides Ken. It seems that a stream of the grain just flows away leaving plywood erosion. Talking to a friend in the building industry he claims that it is a fairly regular issue with old/cheaper ply. I have chosen to sand lightly in an endeavour to smooth out the imperfections rather than I what I regarded as an almost impossible task to block significant areas and destroy the character of the aged plywood. Once finished I have been happy with result and on to the next!

If I understand your issue correctly this is the back of my ball bowler the reflections of which show the timber erosion.

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Skim coat of bondo is the way to go. You could even take it to a body shop and have it done. 10 minutes of work for a bodyman.

Wouldn't this work? No fumes, water based, and inexpensive. Unless there would be some adhesion problem with the old paint on the cab.

" waterbased non-toxic wood filler is the most versatile we've ever used. Straight from the jar it works easily for filling nail holes, dents, and cracks in furniture or flooring. It dries in as little as 15 minutes, exhibits almost no shrinkage, and sets up hard enough to handle machining and fastening just like the wood around it. Slightly thinned with water to the consistency of ketchup, Famowood becomes a superb surface filler, leveling pores and hiding texture very smoothly. It sands beautifully, without loading your paper whether hand or machine sanding. It is fullly compatible with most water- or solvent-based stains & finishes. Famowood is non-combustible & virtually odorless. Colors can be intermixed to create custom shades, & can be further modified with UTCs."

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Go to Walmart and buy a pint can of bondo for about $7.00. While you are there buy a 3-pack of those plastic spreaders. Using a spreader do a a skim coat of bondo as another poster suggested. It will be less work than squeeze tube. But it will still be work.

the products that myself and others have suggested are essentially bondo, only less thick. they're designed for exactly what you're trying to do. using regular bondo or wood putty is going to be much more work and effort-it's just too thick. i'd sand ( you're gonna have to sand anyway) the paint off which will eliminate or reduce some of the cracks and apply glazing putty. this is one of those situations where you're going to have to put the time in and do it right to get the results you'll be happy with.

I found similar results with my first EM cab restore - '67 Bally Rocket III. I kept going and was happy with the end result.
https://pinside.com/pinball/forum/topic/woodgrain-cracks-in-em-cabinet-paint-job

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If you use glazing putty get this, almost all body shops use it

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i use this

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Directions for that says to use a glazing putty over that to finish, might as well just use Bondo

i do not use it on large issues. but on games with a heavy grain, it levels out the grain nicely once sanded. after i sand it i put down my coat or two of sanding sealer then it is smooth as a glass. i still use bondo on large issues and holes but this is what i use on grain issues. the benefit is this is easier to sand and holds better on spots that bondo tends to not hold well like very thin layers.
it also makes a stronger corner than bondo. i have had bondo chip off corners but this stuff is tougher IMO

it has two parts like bondo so your working time can be adjusted a little by cutting back on the activator a little. but that is only a little. to little activator and it will not harden to much and you have little to no working time. i just do small sections at a time.
never used that quick wood but i do use this one.

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