My air compressor runs ok but when I hook up to a air gun or sand blaster . I get about 2 or 3 good blasts and it acts like it out of air. It used to run much better than that. Is there something going out on it. Its about 3 years old

if it is a 11cfm compressor then it produces 11.8 cfm of air regardless of the pressure
pressure has nothing to do with cfm but with the restriction to air flow
for example the tire inflator -air hawk- has a very small cfm but as the tire restrict the air flow the pressure builds up to set pressure on the gauge
so if you want 100psi from the 11.8cfm unit just adjust the pressure switch to shut off at 100psi
Cfm is only important when using air tools as a compressor that is not capable of maintaining the cfm thereby the pressure to run the tools is not a good idea
as an example
a sand blaster that uses 11.8 cfm at 100 psi will not work on your compressor
that unit will require a 20cfm compressor to run it peoperly

you go by how loud it is

2 HP is a std home use direct drive motor 240 volt
with regulator
.. pump tyres . air hose .blow of benches low pressure spray paint gun a rattle gun

step up to twin cylinder belt driven compressors
trade preferred
for high daily use of air tools and spray painting..
8cmf to 10 cmf and more
can high pressure gravity feed spray gun

100 psi is capable of running the gun
what the problem is the volume of air supplied to the gun
check the specification requiring the air usage for the gun against the volume supplied by the compressor
if you gun requires 20 c/ft of air /min and the compressor is only capable of 15c/ft of air /min, then the volume is too low so the pressure will not build up
modern impact 1/2 guns will operate quite well on 80 psi unless the gun is leaking internally
another point is this-- impact guns come in two configurations-- single hammer and double hammer
double hammer give 2 hits for each revolution of the air motor and there fore use less air to do the job
add the fact with 2 hammers , the torque applied is generally higher than single hammer units
the reality is this , the compressor capacity in c/ft/min should be 2 times that required or used by the tool

this is usually caused by:

1. water in pressure tank of the compressor - clean the compressor holding tank
2. faulty valve in direct drive compressor (rare!)
3. dirty needle/gasket in the piece.
4. insufficient air flow

regarding 1:
stop the compressor,
open drain on separator and let all liquid out
allow pressure to drop to atmospheric
open drain valve on the tank and drain the content
on the compressor body find oil drain and drain oil
fill with new oil according to instruction on your compressor
close drain valve on the tank and separator
start compressor and check if problem is solved

re 2:
return valve is too difficult to change without specific model knowledge

re 3:
fill 1/3 (pint/500ml or less container)gun container with paint stripper and spray it directly to the sink or other container on full blast - WEAR SAFETY MASK, GOGGLES AND GLOVES! STRIPPER WILL BURN YOUR SKIN!
fill same level with water or solvent and run again, this operation should remove most of residual paints in nozzles and pipes.
check now nozzle for cleanliness and needle assembly, clean what necessary, use bit of vaseline when you will put it back together.
check for consistency of spray on piece of butcher paper with your usual paint. if problem solved - you are fine!

re4;
increase air flow... this actually should be done first.

for better instruction - please provide with model of the gun and paint used.

your comment and rating will be appreciated.

An air compressor such as the one that you borrowed is very useful for airing-up tires, connecting a nailer for wood working projects and spray painting with a cup gun. All that you need do is plug it in to a 110v outlet and it will fill with air to capacity. To access the air you will need to plug an air hose into a quick coupler and adjust the air pressure with the knob showing two direction arrow. Unless your friend has included an air assisted water blasting wand that would also connect to a water supply, I do not think that it would be useful for cleaning. I have used a compressor such as this with an air nozzle or air blasting the air conditioner condenser fins and the drain to the evaporator inside the house. It will also be really great for cleaning the mower or any outdoor tool that accumulates oily dust (air filters). With a small sand blasting attachment you could remove old paint from the patio furniture or any wood or metal surface needing a good blasting to make it look like new. Sand blasting would not be practical for house/ deck/ fence because it would take too long and use too much sand; then there would be the sand cleanup. Have fun with the compressor and clean-up projects.

hate to say it you need a bigger air compressor

Sounds like your compressor doesn't have the cfm to keep up with the blaster. Blasters use a lot of air. The cfm of your compressor should match or exceed the cfm of the baster. Try a bigger compressor.
Good Luck!

make sure first of all you have a big enough compresor to run ur sand blaster always put clean sand in it,try using a screen if u have to reuse sand if big enough chunks get in the sand it will clog up.hold away from object to be sanded about 3 inches away from area to be sanded.always keep blaster moving if u keep it in 1 spot to long u can warp the metal.other then that i dont know specifically what u need to know.