Maintaining constant velocity while switching on an output

We are currently working on a project in which we apply are applying a patch of spray to a metal surface. This requires getting our CNC machine up to speed then turning on the spray (using the coolant flood command) at a precise position and then turning it off again at a precise position after it has traversed the desired patch. The problem we are having is that the machine slows down in between moves where we turn the spray on. This leads to the edges of the patch receiving a heavier treatment than the rest. I've made sure that G64 is turned on. We are running at relatively high speeds (300 ipm - 600 ipm). We are using a modified Shopbot system with a Mesa 5i25 -7i76 card setup. Is there any way we could do this while maintaining constant velocity? Or is there a way to signal the spray to turn on during the middle of a move? I feel like I'm missing something obvious.

A sample of the G-Code I'm using:

%
G17 G20
G80 G94
G64

G90
G1 Z 0.1 F50

G91
G1 F300

G1 X 3
M8
G1 X 3
M9
G1 X 3
G1 X -3
M8
G1 X -3
M9
G1 X -3
%

Thanks

You will want m62,m63 since m64 and m65 will be queue busters just like m8 and m9.

Thanks for the response. I tried setting up and using the M62 and M63 commands but still have the same issue. The motion slows down significantly between the linear moves.

Look for G64. Probably you demand that stopping.

I increased the acceleration significantly and it seems to have helped but there is still a very clear slow down stutter between moves.

Are you sure you tried it with M62 and M63?
It doesn't seem to slow in my simulation, (It does slow with M64/M65 and M8/M9)

I had been using both M62/M63 and M64/M65 for testing it out. It looks like another reset of the system seems to have helped fix that last little bit of slowdown. Don't know how and it doesn't make me confident that it will stay that way but I'll take it for now. Thank you all for your help.

There is one little thing you must remember about M62/M63. They require that a move be commanded after they are issued for their changes to take effect.