wkchoong, dont be alarmed by the marks. your model will look fairly rough until you go back and wipe it down. here are the steps that i take when doing a wash.
1. after all the paint has been applied to the model i let it cure over night and then spray a couple of coats of future on the model. this puts a protective layer between the harsh thinner wash and your brand new paint job, allows the wash to run more smoothly along the models recesses and such, and makes clean up easier when wiping the excess wash from your subject.
2. this is optional but it looks more realistic, i apply the decals and then seal them with yet another coat of future.
3. mix desired color (mine are usually dark gray or dark umber in color) artist oils in whatever container that you have available ( i use plastic artist palletes from walmart)
4. do like you have said and load the brush and touch the models resesses and raised detail and remember to keep stirring the brew as the artist oils will sink to the bottom very quickly. do the whole model. everything.
5. go chill or work on another model for about an hour or 45 minutes and let the wash dry somewhat. after an hour or so, the thinner should have evaporated and your ready for the cleanup process. i take a q-tip and dip into some clean mineral spirits, roll it around on a paper towel to remove some of the excess thinner, then gently wipe away the excess wash from the model. dont wipe too much though as this will take the wash off if your not careful. this should take care of the brush marks you mentioned and leave the wash where it is intended to be, in the recesses and around raised details. dark umber is gonna be your best bet for a tank as this will give your model a dirty, dingy look. and dont forget to dry brush some highlights with lighter shades of the models base coat. happy modelling. later.