Cracks on polyurethane finish between coats

[YYF]

I was applying the 4th layer of the polyurethane finish to a new benchtop. After it dried, I saw some cracks that didn’t happen in the previous layers. Anyone knows how I can fix them?

https://ibb.co/xgQwnLg
https://ibb.co/xJMFD1h

[Chestnut]

What is the brand and type of poly?

They look like wrinkles you get when a thick finish layer starts to cure. Let the finish cure completely between coats before applying the next. TO fix you'll need to let the finish cure and scrape off the wrinkles or sand off the wrinkles. I'd start with 220 and then move to 320 and then 400 to make the scratches as minimal as possible and then apply another coat. Knowing what poly you are using can help give more advice.

[wtnhighlander]

I think @Chestnut is right, although the streak pattern in the cracks/wrinkles makes me think of contamination on the surface prior to the latest coat. Scraping or sanding back to a smooth layer is the only way to really fix it, in either case. Remember that with poly, each coat should cure hard enough that light sanding with high grit produces fine white powder, and doesn't get gummy on the paper.

[YYF]

Thanks for the answers, guys.

The brand I used is this one: https://www.feastwatson.com.au/products/indoor-products/clear-coat/clear-varnish/

An oil-based matt clear varnish.

I think I probably didn’t allow the coat before the latest cure completely, as I did see my sand paper got gummy.

In terms of sanding and reapplying the finish, should I sand the whole benchtop or just around the wrinkles?

@wtnhighlander I wonder what kind of contamination could it be? Something suspicious I can think of, is the mineral spirits remaining in my brush, or the wet cloth I used to clear the coat before the latest. Could it be any of those?

[wtnhighlander]

Probably just not fully cured on the previous coat. The streaky nature of the wrinkly finish reminded me of finish applied over a surface that had a streak of oil (non-drying type) on it.  I doubt that's it, though. Gummy sandpaper is a sure sign.

[Wimayo]

I generally agree with all of the above comments. I also feel like there could be some kind of contamination. I have seen that problem in my work before and it is usually because of a streak of moisture left after wet sanding. When using an oil based product, I don't use water to wet sand any more. I'm inclined to think that the final wet wipe you did is the cause. Also,did you possibly use a tack cloth to remove dust. Some say they have never had problems using tack cloths. Others say they have and you should never use them. For me, when using an oil based product, I always use a clean rag or towel barely dampened with mineral spirits for the final wipe before applying the next coat. For water based products, I do the same except I dampen with water. It works well for me.