As for your overseeing, get a soil test done first and foremost as well as looking at a core sample. This will help to determine the cause(s) for the problems you are having with the grass that is already established. What is keeping your present grass from looking healthy will in all likelihood prevent the success of your turf renovation.
Ideally you would want to overseed during the late summer – last week of August to around Labour Day, when competition with weeds is of little concern. Regardless of the season (spring or late summer) overseed using the following procedure should give you fairly good success
Rake or power-rake the area(s) to remove debris and to lift the turf leaves.
Aerate twice but ideally 3x, making the 2nd pattern 90 degrees to the 1st, and the 3rd, 45 degrees to the 2nd
Overseed at double the recommended rate of normal seeding, preferably after aerating and certainly before the holes “heal” - seed to soil contact is the key to the over-seeding success.
Fertilize with a “Starter” fertilizer (low nitrogen with higher phosphorus) at the recommended rate
Light water 2 to 3x a day until germination (about a week), then steadily increase amount of water, but with fewer applications
Possible reasons for your lawn’s inability to turn green;
When sand is used in topdressing, there are some considerations to problems that may occur.
Sand decreases microbial activity for thatch and organic decomposition. This is particularly notable when combined with the over-fertilization you suspect the previous owners applied. In short, an excessive thatch layer may have developed
Excessive nutrient leaching, especially nitrogen and potassium
Susceptibility to layering when finer materials such as soil is included with the sand
Although not very likely, there is the possibility of iron deficiency in the soil. When sand is present in the topdressing material, the ph level is increased which in turn decreases the iron availability in the soil. High nitrogen levels (previous owner’s application?) also decreases iron availability.
Hope this helps
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