The issue you are having with metals chipping is more likely to do with the primer not the varnish.
If the primer hasn't bonded with the metal properly then the entire paint (and varnish) is only as good as the weak bond between primer and metal. I used to have this problem – the way I solved it works for me. It may work for you.
Wash metal figures in warm soapy water and scrub with toothbrush. Rinse throughly in hot water. Air dry. Degrease the metal castings. I use cellulose thinners for this. After this don't hold them in your bare hands, use unpowered surgical gloves. Then I soak them for 15 minutes in white vinegar. This is a second round of degreasing but it also very slightly etches the metal not that you can see this.
Then prime with an ACID ETCH primer. This will properly bond into the metal not sit on top of it. Done correctly you will be hard pressed to get the primer off the metal unless you use a sharp metal implement, that's how well a primer should bond. Lots of acrylic car primers will just sit on top of the metal and can easily removed with finger nail pressure. If using acid etch you must spray outside. I've found temperatures of at least 21 degrees C give the best results. Too cold and the primer won't atomise enough – too hot and it dries before it hits the figure. You should be just wafting on thin coats of primer. I now prep everything in summer to build up a winter stock.
Ask any autoshop about painting metal and they will all tell you it has to be degreased first. If not you are only putting paint on top of grease and that is a really poor start for the subsequent paint job.
To most here my method will sound like a real bother and to a degree it is. Equally many/most will say they don't have any chipping problems just using any primer or paint. So their method works for them but currently your method doesn't work for you hence my suggestion as to how I solved the same problem when it happened to me. As with all thees things YMMV.
Having painted I let the figures dry for a minimum of 72 hours, often I leave them for weeks as I have other projects in the pipeline. In 72 hours I swear that Vallejo acrylic slightly changes – I think you can see this. I use Vallejo gloss varnish to seal the paint surface before matt varnish, again a non-yellowing acrylic version.
Without the gloss the matt varnish I use will darken acrylic paint hence using the gloss to seal the surface and preserve the colour. I apply both gloss and matt varnishes with a brush.