It is very difficult to remove grassy weeds or unwanted grass from a KBG lawn. Most herbicides work on either broadleaf (Weed-Be-Gone) or grassy weeds. Seldom do herbicides selectively work on only one type of grass. A pre-emergence will reduce seed germination and helps to remove annual grassy weeds over time from a lawn, but it will do noting for established perennial grassy weeds. Also, there is no pre-emergence labeled for the control of Tall Fescue, Quackgrass or Creeping Bent Grass.
A common problem in KBG lawns is the infiltration of unwanted grasses like Tall fescue (TF). Tall fescue is a coarse bladed perennial bunch grass that's lighter green than KBG. Tall fescue has better heat & drought tolerance than KBG. It will not be crowded out by KBG once established, though it's very slow to spread. Tall fescue looks like a weed in a KBG lawn and is often times confused with crab grass. Tall fescue is green and growing in April (in zone 5) while crab grass has not yet germinated. Crab grass is a warm season annual grassy weed that does not germinate until mid to late May in zone 5. Tall fescue is often times used by DOT (KY31) along roadsides because of its heat & drought characteristics. It spreads to KBG lawns via seeds.
The best technique for the removal of established Tall fesuce is to wipe RoundUp on the blades (use a sponge brush) in early spring. At this time, tall fescue will out grow KBG and its easier to get the RoundUp on the taller, coarser TF blades. Also, because of its better heat tolerance, this can be done in summer when KBG is going dormant due to heat & drought. Multiple applications will be needed to deal a death blow to Tall Fescue. Be careful not to get RoundUp on KBG as it's a nonspecific herbicide & it will kill your KBG. You can pull Tall fescue, but its pretty tough to remove all of the plant if well established. I've had only mixed results pulling Tall fecuse. This grass can regenerate from just a part of the roots.
Quackgrass is a course balded perennial grassy weed that spreads by rhizomes and is often present in KBG lawns. Quackgrass establishes in thin, hot, dry areas of a KBG lawn This could be compacted areas in full sun (along the driveway or narrow sidewalks) or tops and south sides of hills. Use the same technique as described above to remove Tall fescue. Quackgrass can not and should not be pulled. You will never remove all of the rhizomes when pulling this grassy weed. In fact pulling breaks the rhizomes and forces multiple plants to re-establish in its place. It's a fairy tale that you can remove quackgrass via hand pulling.
Creeping Bent grass (CBG) is another grass that can invade a KBG lawn and looks out of place. It's a short, medium fine bladed, dense mat forming grass often used on golf courses. Again, its a grass that can not be controlled by pulling. There is no pre-emergencent herbicide labeled for the control of Creeping Bent Grass. It can be eliminated by digging or using RoundUp. Re-seed or sod the affected area once removed.
Another grass common in KBG lawns are the fine fescues. These are fine bladed perennial and shade tolerant grasses. They may look out of place because their blades are so fine compared to KBG. Thses are usually added to KBG seed mixtures because of their shade tolerance. Seldom do people request the remove of fine fescues.
Don't waste your money on gypsum unless you know that it will work on your lawn. I've succesfully used gypsum on clay when I lived in Phoenix. The University of Ilinois Extension Service advises against the use of gypsum because it has no affect on our type of clays in NE Illinois. Get the facts for your area & check with your local extension office before you make this investment.
Maintaining a dense, lush KBG lawn mowed at 3-3.5" can help keep new seeds from germinating, but it will never crowd out the above grassy weeds/unwanteds if already established in your lawn. Good Luck.