




Calibrated Scale
Pencil grinder or rotary tool (1/2” or 1/4” Carbide burr)
Die grinder (36 grit typ. or diamond grinding wheel)
Finger belt sander

Prior to starting the repair, ensure all required materials and tools are on hand, in good working order and within the
shelf life where applicable.
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TTC Bus Rear Door Corner Crack Repair
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Any deviation from the above list needs to be approved by the dispositioning authority (I.e. Quality
Department/Engineering). Standard lamination tools such as mixing equipment, brushes, rollers etc. are not listed and
shall be used as required. Tools required for disassembly of any cover plates, door sills etc. that aren’t part of the
actual repair aren’t listed here and can be used for such purpose without separate approval.

3 Procedure
3.1 Assessment/Surface Preparation
Obtain quality check sheet and record date and bus number
Take photo of bus identification number located here:

Figure 2 Location of the bus ID

Take photo of rear door corner prior to disassembly
Remove the following parts & hardware:
a. Door panel & associated hardware
b. Threshold heater blanket
c. Auxiliary heat assembly
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AUX HEAT ASSEMBLY

Figure 3 Location of Aux Heat Assembly

Using a small grinding wheel or diamond cutter, create an opening in the threshold/sill where shown:

Figure 4 Drilling location for the crack repair

Sand/grind into the visible gelcoat crack on the side body face using a Rotary tool with a cylindrical file
attachment. Assess the depth of crack on the face and proceed.

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Rotary tools:

Cylindrical File:

Sand outward from the crack using a finger sander with a 36 grit belt, removing the gel coat from the
surrounding area.

Finger (Belt) Sander:

Continue sanding/grinding into laminate until existing crack has been removed and shows no further
propagation. Void or cavity between bonded parts (floor & body) may be exposed when sanding through
the laminate.

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Figure 5 Excavated corner crack

Feather/taper the sanding outward from the exposed edges the valleys or crack center.
If any wet balsa is observed by hand in the voids, extract the wet balsa, and allow the cavity to dry
overnight or accelerate the drying via a heat gun and air movement.
Vacuum or blow out any debris in the voids; vacuum all surface debris and use a clean rag with acetone
to remove any remaining dust.
Repair area grinding complete. Record photos of repair area.

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3.2 Crack Repair
Ensure all area of exposed surface is clean of dust, debris, grease or any type of surface contamination.
Repeat step 3.1.11 if necessary.
Ensure to fill any voids or cavities in the excavated area by using VE chopped fiber putty (No. 876).
Required hardener quantity for the VE chopped fiber putty is 2.0% by mass.

Putty filled state

Figure 6 Filling the exposed cavity with VE chopper fiber putty (No. 876)

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Wet out 1 layer of 1708 Biaxial glass with catalyzed VE resin (Hydropel H034-AKA-33 with MEKP 925,
typical for all plies listed in this repair) and run the ply from the threshold onto the door jamb. Fiber
direction of ±45° is running along the length of the strip. Catalyst mass shall be 1.0% – 2.0% of the total
resin mass (e.g. 100 g resin = 1 – 2 g of catalyst). It is recommended that the total mix volume does not
exceed 100 g.

Fiber orientation

Figure 7 Application of first reinforcement ply (1708)

Fill any valleys using short strips of 1708 biaxial glass as shown in the side view (left) and top view
(right) in the figure below; add just enough layers of 1708 to help level the void with the surrounding
intact laminate skins. Strips should gradually increase in size, as needed, but should remain in the same
plane and not be directed around corners. Record photo of glass application.

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Figure 8 Schematics of filling the “valley” with reinforcement plies (1708)

Alternate 1708 plies from front to back with those running from the sill onto the jamb. Each ply must get
bigger than the previous one so that they extend beyond the previous one. All plies have ±45° fiber
direction running along the long side of the strip. Record photo of glass application.
1708 layers, 3 plies in each direction (total of 6 plies) that are “woven” together:
a.

1st ply from the front side to the back side.

b.

1st ply from sill to jamb.

c.

2nd ply 15 to 20 mm longer than the 1st (at each end) from front to back.

d.

2nd ply 15 to 20 mm longer than the 1st (at each end) from sill to jamb.

e.

3rd ply 15 to 20 mm longer than the 2nd (at each end) from front to back.

f.

3rd ply 15 to 20 mm longer than the 2nd (at each end) from sill to jamb.

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Figure 9 Application of reinforcement plies on the door corner (1708)

If needed, add additional plies of 1708 glass to the threshold/sill to obtain necessary height and width.
Stagger the plies so that the ends extend further than the previous ply.
Add 6 plies of 1708 on the backside as shown; start at the corner and overlay the previously installed
repair plies. Each additional ply should be 15 to 20mm larger than the previous ply, but only in the the
directions as shown in the image below. Ensure to roll each layer with a roller to smooth out the glass
and remove any air bubbles. Record photos of glass application at each stage.

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Fiber orientation

Figure 10 Application of reinforcement plies on the backside of the door

Upon completing the repair, allow to cure for minimum 15 minutes. Record photos of completed repair.

3.3 Gelcoat Patching:
Ensure the faired area is clean of dust and debris, as well as grease by performing solvent (e.g. acestone)
wipe.
Fill in pin holes or discontinuous surfaces using No. 01175 lightweight putty (putty No. 01175 light
weight body filler), catalyzed with cream hardener 10140. Mix ratio is 100 g filler to 3 ± 0.5 g hardener.
Allow to cure 15-25 minutes before sanding.
Fair the patched area using a DA sander with 80 or 120-grit sanding disk.
Sand the surfaces around the repair with 220 grit on the DA for adhesion and blending of the new gelcoat.

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Figure 11 Filled and faired surface of the repair

Mask the area surrounding patch, using craft paper and green masking tape. The masking should start
just inside the area sanded with 220 grit.

Figure 12 Masking the repair area prior to gel coating

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Mix 970C961 Spray Cure into the gelcoat at a ratio of 16 oz. per gallon (128 oz.) of Polynt 99F gelcoat.
The Spray Cure allows the surface of the sprayed gelcoat to cure fully.
Polynt 99F gelcoat quantity
970C961 spray cure quantity
MEKP 925 catalyst quantity
Sample
mix #1

5.3 oz (150 g)

0.7 oz (18.8 g)

0.09 ~ 0.10 oz (2.5 ~ 2.9g)

Sample
mix #2

10.6 oz (300 g)

1.4 oz (37.6 g)

0.18 ~ 0.20 oz (5.0 ~ 5.7g)

Sample
mix #3

15.9 oz (450 g)

2.1 oz (56.4 g)

0.27 ~ 0.30 oz (7.6 ~ 8.6 g)

Prepare either gel coat patch gun or Preval Sprayer.
Catalyze the gelcoat at a ratio of 10-15 cc MEKP 925 per quart – depending on shop temperature. (High
temp. = less MEKP). Refer to the table in 3.3.7.
Perform a test spray on scrap kraft paper away from part. Spray should be uniform in consistency (e.g. no
“spitting” or dripping) and color.
Apply the first coat of gelcoat to the area of patch, keeping a perpendicular distance of approx. 12”.

Figure 13 Application of gel coat

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Spray 3 more coats with about a minute between each coat. Each coat should cover the repair and extend
farther out toward the tape than the previous coat.
Allow gel coat to fully cure for approx. 45 minutes.
Remove masking and inspect.

Figure 14 Completed repair

Fair area using DA with 320 grit & 600 grit sanding disks. Take photos of the finished repair.

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