Eventually a painting will need to be cleaned. Dust, grime, and the usual natural absorption of the first original varnish layer by the paint layer will soon require a light dusting and re-varnishing. This is usually recommended when the painting is still fairly new - within the first 10 years.
There are always some exceptions but usually, after cleaning, the painting is revarnished to protect the surface. (The surface reflectance of the final varnish can be adjusted by the conservator to make the painting look either glossy, semi-matte (satin), or completely dead-matte and unvarnished if necessary.
If your canvas is torn, or your panel is cracked, or the paint is beginning to lift off the surface you need structural repairs. Small to medium sized tears can be carefully re-welded and re-woven. Cracking or broken panel paintings will be expertly treated by specialists with woodworking skills - this is a real art. Raised cracking through the paint layer can be consolidated with the proper adhesives that will not stain the original colors or change the characteristics of the painting. Larger tears in canvas paintings will require lining - an addition of a second canvas to the reverse using a proper lining adhesive.
There is no way to "quickly" fix or clean & re-varnish a painting. All of the above takes time. And it takes time to do well. Conservators charge by the hour. Generally cleaning and revarnishing even a small painting will take 2-5 hours since the painting has to be removed from the frame, photographed, tested, reframed correctly after cleaning, & photographed on completion.
Tips & Suggestions
Conservators are not out there deciding what gets treated and what doesn't. Due to the costs involved, you, the owner, have to decide what to invest your money in. Generally the prices of art conservation are very reasonable considering the longevity of professional conservation treatments and the high value of collectable paintings and period frames. If you do not know anything about your piece have it appraised before you bring it in for conservation.