Haven't tried myself but I guess it should be possible to brush it, as it does not need to be perfectly even.

Not sure about thinning, it may loose its properties...

According to the product description on the Vallejo site, it can be
airbrushed or brushed on.

As far as thinning is concerned, it probably thins with some water.
Of course you want to test that first on some scrap plastic before
actually using it on your prize model.

Cheers,

Vallejo chipping medium, can it be brushed on?

What can it be thinned with besides Vallejo thinner?