There are many chemicals in market. You can use those as per the literature available. But, main problem is to find the source of dampness. If you do not judge the source of dampness you can not have a permanent solution. The reasons for dampness are many,
1. During monsoon,
2. Through out the year.
If the dampness is seen only during the monsoon problem is little simple. It must be from out side of the home. reasons here again are ample,
1. outside wall cracks,
2. from terrace,
3. from joints of chajjas and extentions,
4. from inlet pipes and cables which are not sealed properly, or any other which can be found on actual site visit by an expert.
If dampness is perineal i.e. through out the year,
Problem is little complicated. Reasons can be,
1. upper floor leakages in Wet areas i.e. Toilet, wash basin and kitchen areas.
2. It can be due to saline (salty) sand used during construction for internal plaster,
3. If the home is on ground floor, the underground watr may comeup due to capilary action,
4. the water from wet areas of the same home can also be one of the cause, if you see dampness upt few feet from flooring.
This way there can be ample reasons for the dampness. One has to find the source and give exact treatment needed for it according to the source and cause.
The best way is to call experienced water proofing expert and try the solution suggested. and do work under his guidance.