The scratching and chipping depend on the coat type. The oil enamels are more resistant to the type of damging usually. Contrary to that the acrylic paints are more sensitive to. To be honest the coat of varnish , even if it is a clear one but is of the acrylic one, may be not enough to protect the botom surfaces. What is more , a such applying is a double work I would say. The same you have to do for the top one. As a result you have to change the sprayed coats at least four times and each time you will have to clean the airbrush removing the residues of colours.
Majority of guys here use special supports that can be buy in local hobby shops or via the net.
Persanally I use a pices of a styrofoam as a base for painting. For handling of a model I use a round file put into a hole in the nose of a model where the spinner/prop shaft is going to be attached. In the way I can operate the model easily and work on each of sides. When I need to put it away I use a cardboard box ( it can be the one of the kit ) and gently put it upside down in the way the model wings are supported by the top edges of the side walls of the box until the coat of the undersides is cured fully. Then I can work on tops still handling and operating the model with the file. If I need to put it away again I just gently put the model on the box supporting the wing undersides at the edges of the side walls.
The procedure for the models of larger scales is a little bit different...
1. I mask all details that don't need to be painted firstly ....
2. I push a couple of toothpicks into slots for landing gear legs or other available holes there ....
3. Then I apply the colour of undersides and put the model away impacting the toothpicks a little bit into the styrofoam. In the way I get the support of the model without the risk of damaging the bottom surfaces ....
4. Moreover , I can rotate the stable :standing" model with the base together and work around tops ....
5. Sometimes I use a roll of a masking tape as the support , overlain with a soft paper towel. This allows to rotate the model as well ....
6. When model is coated I wrap the fuselage at its rear area with a clean soft rag. It allows me to handle and operate the model securely ....