If at any point in time, there has been a sealer or paint that is blocking your concretes pores it has to be removed. Mechanically grinding the surface will remove coatings and such while also opening up your concrete’s pores. The test for sealers, if you’re not sure, can be done by simply sprinkling water on the surface and checking to see if the water was absorbed. Concrete is an extremely porous surface and will allow water to penetrate its pores if there is not a sealer or paint on the surface. Sometimes, it can be hard to tell with the water test, so the next possible option is an acid etch. Apply some acid in the area you’re trying to test for sealer, if it turns yellow and fizzy then you do not have any sealer on your concrete but if there is no reaction at all then there is a sealer that needs to be removed.