Trail conditions???

Is the trail too sloppy for fenders? I have 3/4 inch clearance for tires.

is 38 mm gravel king (slick) sufficient?

Heading there in 9/13/21 to do C & o GAP

Under wet, rainy conditions fenders will be fine on the GAP and much of the C&O, especially the restored sections of the C&O. The fenders would obviously reduce the amount of trail grit and mud on your shoes and lower legs...as well as your back.

With 3/4" clearance between your tires and fenders, you should be able to traverse the C&O successfully. There would usually be enough water in standing puddles in the tracks to partially flush the mud. At worst you may want/need to stop at some of the trailside hiker/biker campsites with functioning pumps to use a water bottle to help spray some water up under the fenders, especially at day's end so the mud does dry underneath the fenders before starting the next day. If your are diligent and stealthy enough, you'll also find places such as hotels, convenience stores, gas stations, banks, etc. with a hose to flush out your fenders.

As a suggestion, you should probably not try to skirt around the perimeters of the puddles in the tracks to avoid the water. The sides of the puddles become quite slick and add to the risk of crashing should the tires slide back into the bottom of the puddle. I recommend you simple grin and bear it and ride straight through the puddles. You 38mm tires should be ok in doing so but care is always advised.

I second Rivnuts opinion to just plow through the puddles. Fenders or no fenders, you are going to get muddy when the C&O is wet, there's just no way to avoid the majority of it.

I guess a "glass half full" outlook on it is that the mud adds to the "adventure" and you'll have a great story to look back on as well as a huge sense of accomplishment that you made it through.