Found an old 2mm sheet of timber (plywood?). It has 3 layers and some parts are coming apart. Got my friend to cut me strips to use as comb guides. Before I put it into the brood and honey boxes, thought I get some opinion if it’s ok to use old wood.
Should be fine. I might glue it back together with some PVA glue first, clamping it tightly for up to 24 hours. It would last longer then. 
As @Dawn_SD says and I agree, you can use PVA glue to rejoin it back together if you really want to go foundationless, but a better option would be to glue in, or wax in, strips of foundation wax. But you can still have issues with wonky comb even if you use a spirit level to make sure the hive is dead flat.
Yes, I have been there and tried that, but for me the best way of having comb made with no hassles is by using a full sheet of foundation and wiring it in.
Cheers
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Next spring I will be using foundation. Right now trying out foundationless plus hoping to get excess honeycomb. Next year will be flow super for both my hives. 
Bees always prefer old wood. But it’s ok to use new wood if you don’t have old wood. 
I googled plywood and apparently the glue has formaldehyde. I am guessing since the layers are coming apart, the glue must have disintegrated (not sure it’s the right word) and less toxic now?
Nice upcycling.
#onlyoneplanet
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I think wood comb guides make great garden markers, frame spacers before transporting nucs, smoker fuel or fire kindling. Like @Peter48, I prefer properly fitted full sheets of wax foundation.
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Formaldehyde was used in the glue for ply wood but I think that is now obsolete. Maybe a year ago I emailed a plywood manufacturer in Australia and asked and explained what I wanted to use the plywood for but of course, no reply received.
Cheers
I do like real wood, but my covers and bottoms are mostly plywood. They don’t make boards wide enough… I don’t think they use formaldehyde anymore in the glue. There was a lot of flack back 20 or so years ago about all the chemicals in new buildings.
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Thank you everyone for your replies. I will use it for a few frames. I will be trying out foundation as well. Being a newbee, I thought I get a feel on the different frames. Maximising honey is not my main objective at the moment. Learning and experimenting. Early next year going to do a vertical split to raise a new queen. Current one is almost 3yo and slowing down. That should be exciting.
I don’t know if you can buy it in the US but Weathertex is great from a replacement for ply wood for base board and roofs. I have been using it for well over 10 years with no issues in any way and the price is good. I changed to that as a replacement for ply wood, even when painted the glue would break down and de-laminate.