Okay Richard here we go....You asked for it!!!
In weathering model boats there are many techniques available, I just find that to make a vessel look used this method works. Is simple to do, and any errors can be cleaned up, leaving the finished result just as you want it. It is very effective on both plastic smooth finishes or wood.
Why milk? Well unlike water, milk will allow the paint to be removed with a damp cloth even when dry.
Preparation is very important. After final painting, ensure everthing to be weathered has a coat of Matt varnish and allow to dry thouroughly. Next make sure you have an hour or two to sit in one go. Mix a small quantity of milk in a glass and add a few drops of water based black paint to it and mix it together well. You should have a good black and not grey colour. Have a good supply of fresh water to hand together with a clean damp cloth and a couple of different size paint brushes.
Now do not cover too much at once until you have had some practice, but cover the area liberally as shown in the photos and leave for a few minutes to "dry". I always find it easier to start removing the mixture before it is fully dry. Using the damp cloth wipe the paint off. initially try to wipe it all off. You will find that some residue stays in the joints and cracks and the grain of any wood used. In the difficult to get at areas you can use the pointed end of a paint brush wrapped in the cloth. Keep cleaning the cloth out as much as is needed. To give a dirtier effect simply do not clean up as much.
If you find you have over done it, use a paint brush and water to release the mixture and wipe the excess off.
When happy with the final effect, leave to dry properly and then double check that all is well before sealing it in with another coat of Matt varnish.
Do not use on unvarnished decks and fittings made of timber...
Barry