hi Helena,
You can not fix old nylon/plastic chopping boards they are designed that once damegaed you throw away or give it to a woodworker as old nylon/plastic boards are great to make sliders out of and other bits and bobs for the workshop Or you could use them to make a garden border. As that is all they are good for once past their used by date.
You can not put them through a plainer you will damage your plainer and the chopping board, besides the chopping boards were designed to have a dimpled surface so that will not slide on the bench top or allow the food to slide around too much if kept dry when cutting meat that is.
I mentioned a use by date above this is correct you can not use them for ever like the wooden boards, they need to be replaced every few years due to the knifes interaction with the surface and over time if the plastic board is not thick enough it develops a concave shape to it and this is one of the other reasons the plastic/nylon chopping boards have to be replaced. This goes the same with the “SoderStream” bottles if you checked the small print on the bottle it will have a date on it for when you need to replace the bottles.
If you have a dishwasher you can put in there to wash I do that with mine it’s okay but I only use it to cut the cats meat on. In the food industry they use the plastic/nylon boards because they can go into the dishwasher and this will sterlize them and is the only reason they use those types of chopping boards now and not the wooden ones. Wooden chopping boards can not go into the dishwasher as they are glued together with food safe glue and with the dishwasher heating up this softnes the glue to the point the boards fall apart. I know I did it with my own chopping board I had :-) it was a lovely mess to be cleaned out of the dishwasher lol
As for my food prep area I have a wooden chopping board that’s been in the family for 40 odd years now. It was me Mom’s chopping board and when she passed away I kept it as it is a good quality one chopping board and it’s easy to keep clean. (As I had killed my board in the dishwasher lol)
Even with the number of years On it it’s still as good as new as we have always had it has sanded Every 6-8 months and I have even put it through the plainer to flatten it out a bit as it is almost 3” thick I don’t think I have to worry too much about it :-) but it does need a light sanding at least twice a year to clean it off in preparation for food safe coating and that’s not cooking oil as it goes sticking with age, you need the correct coating which I managed to get from the hardware store :-)
I hope this helps :-)