How to paint on dried oil layer?

I want to paint over a dried oil painting which I have not finished. Do I need to apply some linseed oil or something like that before I paint new layer oil on it? and why? Thanks!How to paint on dried oil layer?
You can paint right over it, but your new layer needs to have more oil in it that the previous one did, in order to bind to it properly.





There is also something called ';retouch varnish'; which is applied all over a dried layer, to provide a uniform surface and finish for adding new details to a dried painting.How to paint on dried oil layer?
Well I've had to do this on several occasions and what I did was to figure out a very opaque color of paint, in my case it was ';unbleached titanium white. I had to very lightly sand the areas of the old painting to make sure it didnt show through the new painting. The titanium white works wonderfully as a basecoat and I will give you my little formula for making sure the paint dries fast enough for me to use the canvas asap,.....mix one bottle of cobalt drier with one bottle of the same size of varnish, then take one of those empty bottle and fill it three times and mix it with the drier %26amp; the varnish into a bottle or jar,.......next, take one tablespoon of ';laquer thinner'; and add it to the mixture. The laquer thinner acts as a catalyst when you dip your brush in the mixture and then pull it out, the air hits the paint and the catalyst (laquer thinner) acts to activate the mixture you just made, your oils should dry and about 20 to 30 minutes...there will be no cracking and the paint will adhere better to the canvas......................good luck to you and happy painting.