<p>I have a 3D printer at home, the Colido Compact, and for some reason when I 3D print big flat surfaces a really weird thing happens. I'm using some PLA from Colido too I think</p>

<p><a href="https://i.stack.imgur.com/QOMPF.jpg" rel="nofollow noreferrer" title="Weeeird surface artefacts"><img src="https://i.stack.imgur.com/QOMPF.jpg" alt="Weeeird surface artefacts" title="Weeeird surface artefacts"></a></p>

<p>It almost seems as if the bottom layer does perfectly but when it starts printing out the top surface this happens, because the one on the bottom left is in two parts because the upper part is the bottom one and that one is perfectly flat, then I took them apart and the weird thing just stayed with the top part... and also on the weird warps there are bits of brown goo or something? I don't know it looks as if the filament was burned...
It only appears on pretty big surfaces because smaller ones don't seem to have the problem.</p>

<p>Anyone knows what is happening?</p>

The oozing is due to hot-end getting hot before the bed leveling procedure: if you move the hot-end warm up command
after
the
```
G29
```
line you avoid that oozing
```
```
; Ender 3 Custom Start G-code
M104 S{material_print_temperature_layer_0} ; Set Extruder temperature
M140 S{material_bed_temperature_layer_0} ; Set Heat Bed temperature
G28 ; Home all axes
G29 ; BLTOUCH Mesh Generation
M190 S{material_bed_temperature_layer_0} ; Wait for Heat Bed temperature
M109 S{material_print_temperature_layer_0} ; Wait for Extruder temperature
G1 F1800 E-3 ; Retract filament 3 mm to prevent oozing
G92 E0 ; Reset Extruder
G1 Z5.0 F3000 ; Move Z Axis up little to prevent scratching of Heat Bed
G1 X0.1 Y20 Z0.3 F5000.0 ; Move to start position
G1 X0.1 Y200.0 Z0.3 F1500.0 E15 ; Draw the first line
G1 X0.4 Y200.0 Z0.3 F5000.0 ; Move to side a little
G1 X0.4 Y20 Z0.3 F1500.0 E30 ; Draw the second line
G92 E0 ; Reset Extruder
G1 Z5.0 F3000 ; Move Z Axis up little to prevent scratching of Heat Bed
```
```
The above code will activate the heating elements but starts homing and leveling procedure without waiting for the elements to get up to temperature. Only after the bed leveling is finished the printer will pause and wait for the heating elements reach the desired temperature.
This will prevent oozing on a cold start, but you will still be affected if you start a print right after another print, when the hot-end is still close to  melting temperature.
If you prefer to avoid that condition you might want to also move the
```
M104
```
and
```
M140
```
commands after the
```
G29
```
bed leveling command.